[ceph.git] / ceph / src / crush / mapper.c

/*
 * Ceph - scalable distributed file system
 *
 * Copyright (C) 2015 Intel Corporation All Rights Reserved
 *
 * This is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License version 2.1, as published by the Free Software
 * Foundation.  See file COPYING.
 *
 */

#ifdef __KERNEL__
# include <linux/string.h>
# include <linux/slab.h>
# include <linux/bug.h>
# include <linux/kernel.h>
# include <linux/crush/crush.h>
# include <linux/crush/hash.h>
#else
# include "crush_compat.h"
# include "crush.h"
# include "hash.h"
#endif
#include "crush_ln_table.h"
#include "mapper.h"

#define dprintk(args...) /* printf(args) */

/*
 * Implement the core CRUSH mapping algorithm.
 */

/**
 * crush_find_rule - find a crush_rule id for a given ruleset, type, and size.
 * @map: the crush_map
 * @ruleset: the storage ruleset id (user defined)
 * @type: storage ruleset type (user defined)
 * @size: output set size
 */
int crush_find_rule(const struct crush_map *map, int ruleset, int type, int size)
{
        __u32 i;

        for (i = 0; i < map->max_rules; i++) {
                if (map->rules[i] &&
                    map->rules[i]->mask.ruleset == ruleset &&
                    map->rules[i]->mask.type == type &&
                    map->rules[i]->mask.min_size <= size &&
                    map->rules[i]->mask.max_size >= size)
                        return i;
        }
        return -1;
}

/*
 * bucket choose methods
 *
 * For each bucket algorithm, we have a "choose" method that, given a
 * crush input @x and replica position (usually, position in output set) @r,
 * will produce an item in the bucket.
 */

/*
 * Choose based on a random permutation of the bucket.
 *
 * We used to use some prime number arithmetic to do this, but it
 * wasn't very random, and had some other bad behaviors.  Instead, we
 * calculate an actual random permutation of the bucket members.
 * Since this is expensive, we optimize for the r=0 case, which
 * captures the vast majority of calls.
 */
static int bucket_perm_choose(const struct crush_bucket *bucket,
                              struct crush_work_bucket *work,
                              int x, int r)
{
        unsigned int pr = r % bucket->size;
        unsigned int i, s;

        /* start a new permutation if @x has changed */
        if (work->perm_x != (__u32)x || work->perm_n == 0) {
                dprintk("bucket %d new x=%d\n", bucket->id, x);
                work->perm_x = x;

                /* optimize common r=0 case */
                if (pr == 0) {
                        s = crush_hash32_3(bucket->hash, x, bucket->id, 0) %
                                bucket->size;
                        work->perm[0] = s;
                        work->perm_n = 0xffff;   /* magic value, see below */
                        goto out;
                }

                for (i = 0; i < bucket->size; i++)
                        work->perm[i] = i;
                work->perm_n = 0;
        } else if (work->perm_n == 0xffff) {
                /* clean up after the r=0 case above */
                for (i = 1; i < bucket->size; i++)
                        work->perm[i] = i;
                work->perm[work->perm[0]] = 0;
                work->perm_n = 1;
        }

        /* calculate permutation up to pr */
        for (i = 0; i < work->perm_n; i++)
                dprintk(" perm_choose have %d: %d\n", i, work->perm[i]);
        while (work->perm_n <= pr) {
                unsigned int p = work->perm_n;
                /* no point in swapping the final entry */
                if (p < bucket->size - 1) {
                        i = crush_hash32_3(bucket->hash, x, bucket->id, p) %
                                (bucket->size - p);
                        if (i) {
                                unsigned int t = work->perm[p + i];
                                work->perm[p + i] = work->perm[p];
                                work->perm[p] = t;
                        }
                        dprintk(" perm_choose swap %d with %d\n", p, p+i);
                }
                work->perm_n++;
        }
        for (i = 0; i < bucket->size; i++)
                dprintk(" perm_choose  %d: %d\n", i, work->perm[i]);

        s = work->perm[pr];
out:
        dprintk(" perm_choose %d sz=%d x=%d r=%d (%d) s=%d\n", bucket->id,
                bucket->size, x, r, pr, s);
        return bucket->items[s];
}

/* uniform */
static int bucket_uniform_choose(const struct crush_bucket_uniform *bucket,
                                 struct crush_work_bucket *work, int x, int r)
{
        return bucket_perm_choose(&bucket->h, work, x, r);
}

/* list */
static int bucket_list_choose(const struct crush_bucket_list *bucket,
                              int x, int r)
{
        int i;

        for (i = bucket->h.size-1; i >= 0; i--) {
                __u64 w = crush_hash32_4(bucket->h.hash, x, bucket->h.items[i],
                                         r, bucket->h.id);
                w &= 0xffff;
                dprintk("list_choose i=%d x=%d r=%d item %d weight %x "
                        "sw %x rand %llx",
                        i, x, r, bucket->h.items[i], bucket->item_weights[i],
                        bucket->sum_weights[i], w);
                w *= bucket->sum_weights[i];
                w = w >> 16;
                /*dprintk(" scaled %llx\n", w);*/
                if (w < bucket->item_weights[i]) {
                        return bucket->h.items[i];
                }
        }

        dprintk("bad list sums for bucket %d\n", bucket->h.id);
        return bucket->h.items[0];
}


/* (binary) tree */
static int height(int n)
{
        int h = 0;
        while ((n & 1) == 0) {
                h++;
                n = n >> 1;
        }
        return h;
}

static int left(int x)
{
        int h = height(x);
        return x - (1 << (h-1));
}

static int right(int x)
{
        int h = height(x);
        return x + (1 << (h-1));
}

static int terminal(int x)
{
        return x & 1;
}

static int bucket_tree_choose(const struct crush_bucket_tree *bucket,
                              int x, int r)
{
        int n;
        __u32 w;
        __u64 t;

        /* start at root */
        n = bucket->num_nodes >> 1;

        while (!terminal(n)) {
                int l;
                /* pick point in [0, w) */
                w = bucket->node_weights[n];
                t = (__u64)crush_hash32_4(bucket->h.hash, x, n, r,
                                          bucket->h.id) * (__u64)w;
                t = t >> 32;

                /* descend to the left or right? */
                l = left(n);
                if (t < bucket->node_weights[l])
                        n = l;
                else
                        n = right(n);
        }

        return bucket->h.items[n >> 1];
}


/* straw */

static int bucket_straw_choose(const struct crush_bucket_straw *bucket,
                               int x, int r)
{
        __u32 i;
        int high = 0;
        __u64 high_draw = 0;
        __u64 draw;

        for (i = 0; i < bucket->h.size; i++) {
                draw = crush_hash32_3(bucket->h.hash, x, bucket->h.items[i], r);
                draw &= 0xffff;
                draw *= bucket->straws[i];
                if (i == 0 || draw > high_draw) {
                        high = i;
                        high_draw = draw;
                }
        }
        return bucket->h.items[high];
}

/* compute 2^44*log2(input+1) */
static __u64 crush_ln(unsigned int xin)
{
        unsigned int x = xin;
        int iexpon, index1, index2;
        __u64 RH, LH, LL, xl64, result;

        x++;

        /* normalize input */
        iexpon = 15;

        // figure out number of bits we need to shift and
        // do it in one step instead of iteratively
        if (!(x & 0x18000)) {
          int bits = __builtin_clz(x & 0x1FFFF) - 16;
          x <<= bits;
          iexpon = 15 - bits;
        }

        index1 = (x >> 8) << 1;
        /* RH ~ 2^56/index1 */
        RH = __RH_LH_tbl[index1 - 256];
        /* LH ~ 2^48 * log2(index1/256) */
        LH = __RH_LH_tbl[index1 + 1 - 256];

        /* RH*x ~ 2^48 * (2^15 + xf), xf<2^8 */
        xl64 = (__s64)x * RH;
        xl64 >>= 48;

        result = iexpon;
        result <<= (12 + 32);

        index2 = xl64 & 0xff;
        /* LL ~ 2^48*log2(1.0+index2/2^15) */
        LL = __LL_tbl[index2];

        LH = LH + LL;

        LH >>= (48 - 12 - 32);
        result += LH;

        return result;
}


/*
 * straw2
 *
 * for reference, see:
 *
 * http://en.wikipedia.org/wiki/Exponential_distribution#Distribution_of_the_minimum_of_exponential_random_variables
 *
 */

static inline __u32 *get_choose_arg_weights(const struct crush_bucket_straw2 *bucket,
                                            const struct crush_choose_arg *arg,
                                            int position)
{
  if ((arg == NULL) ||
      (arg->weight_set == NULL) ||
      (arg->weight_set_size == 0))
    return bucket->item_weights;
  if (position >= arg->weight_set_size)
    position = arg->weight_set_size - 1;
  return arg->weight_set[position].weights;
}

static inline int *get_choose_arg_ids(const struct crush_bucket_straw2 *bucket,
                                        const struct crush_choose_arg *arg)
{
  if ((arg == NULL) || (arg->ids == NULL))
    return bucket->h.items;
  return arg->ids;
}

static int bucket_straw2_choose(const struct crush_bucket_straw2 *bucket,
                                int x, int r, const struct crush_choose_arg *arg,
                                int position)
{
        unsigned int i, high = 0;
        unsigned int u;
        __s64 ln, draw, high_draw = 0;
        __u32 *weights = get_choose_arg_weights(bucket, arg, position);
        int *ids = get_choose_arg_ids(bucket, arg);
        for (i = 0; i < bucket->h.size; i++) {
                dprintk("weight 0x%x item %d\n", weights[i], ids[i]);
                if (weights[i]) {
                        u = crush_hash32_3(bucket->h.hash, x, ids[i], r);
                        u &= 0xffff;

                        /*
                         * for some reason slightly less than 0x10000 produces
                         * a slightly more accurate distribution... probably a
                         * rounding effect.
                         *
                         * the natural log lookup table maps [0,0xffff]
                         * (corresponding to real numbers [1/0x10000, 1] to
                         * [0, 0xffffffffffff] (corresponding to real numbers
                         * [-11.090355,0]).
                         */
                        ln = crush_ln(u) - 0x1000000000000ll;

                        /*
                         * divide by 16.16 fixed-point weight.  note
                         * that the ln value is negative, so a larger
                         * weight means a larger (less negative) value
                         * for draw.
                         */
                        draw = div64_s64(ln, weights[i]);
                } else {
                        draw = S64_MIN;
                }

                if (i == 0 || draw > high_draw) {
                        high = i;
                        high_draw = draw;
                }
        }

        return bucket->h.items[high];
}


static int crush_bucket_choose(const struct crush_bucket *in,
                               struct crush_work_bucket *work,
                               int x, int r,
                               const struct crush_choose_arg *arg,
                               int position)
{
        dprintk(" crush_bucket_choose %d x=%d r=%d\n", in->id, x, r);
        BUG_ON(in->size == 0);
        switch (in->alg) {
        case CRUSH_BUCKET_UNIFORM:
                return bucket_uniform_choose(
                        (const struct crush_bucket_uniform *)in,
                        work, x, r);
        case CRUSH_BUCKET_LIST:
                return bucket_list_choose((const struct crush_bucket_list *)in,
                                          x, r);
        case CRUSH_BUCKET_TREE:
                return bucket_tree_choose((const struct crush_bucket_tree *)in,
                                          x, r);
        case CRUSH_BUCKET_STRAW:
                return bucket_straw_choose(
                        (const struct crush_bucket_straw *)in,
                        x, r);
        case CRUSH_BUCKET_STRAW2:
                return bucket_straw2_choose(
                        (const struct crush_bucket_straw2 *)in,
                        x, r, arg, position);
        default:
                dprintk("unknown bucket %d alg %d\n", in->id, in->alg);
                return in->items[0];
        }
}

/*
 * true if device is marked "out" (failed, fully offloaded)
 * of the cluster
 */
static int is_out(const struct crush_map *map,
                  const __u32 *weight, int weight_max,
                  int item, int x)
{
        if (item >= weight_max)
                return 1;
        if (weight[item] >= 0x10000)
                return 0;
        if (weight[item] == 0)
                return 1;
        if ((crush_hash32_2(CRUSH_HASH_RJENKINS1, x, item) & 0xffff)
            < weight[item])
                return 0;
        return 1;
}

/**
 * crush_choose_firstn - choose numrep distinct items of given type
 * @map: the crush_map
 * @bucket: the bucket we are choose an item from
 * @x: crush input value
 * @numrep: the number of items to choose
 * @type: the type of item to choose
 * @out: pointer to output vector
 * @outpos: our position in that vector
 * @out_size: size of the out vector
 * @tries: number of attempts to make
 * @recurse_tries: number of attempts to have recursive chooseleaf make
 * @local_retries: localized retries
 * @local_fallback_retries: localized fallback retries
 * @recurse_to_leaf: true if we want one device under each item of given type (chooseleaf instead of choose)
 * @stable: stable mode starts rep=0 in the recursive call for all replicas
 * @vary_r: pass r to recursive calls
 * @out2: second output vector for leaf items (if @recurse_to_leaf)
 * @parent_r: r value passed from the parent
 */
static int crush_choose_firstn(const struct crush_map *map,
                               struct crush_work *work,
                               const struct crush_bucket *bucket,
                               const __u32 *weight, int weight_max,
                               int x, int numrep, int type,
                               int *out, int outpos,
                               int out_size,
                               unsigned int tries,
                               unsigned int recurse_tries,
                               unsigned int local_retries,
                               unsigned int local_fallback_retries,
                               int recurse_to_leaf,
                               unsigned int vary_r,
                               unsigned int stable,
                               int *out2,
                               int parent_r,
                               const struct crush_choose_arg *choose_args)
{
        int rep;
        unsigned int ftotal, flocal;
        int retry_descent, retry_bucket, skip_rep;
        const struct crush_bucket *in = bucket;
        int r;
        int i;
        int item = 0;
        int itemtype;
        int collide, reject;
        int count = out_size;

        dprintk("CHOOSE%s bucket %d x %d outpos %d numrep %d tries %d \
recurse_tries %d local_retries %d local_fallback_retries %d \
parent_r %d stable %d\n",
                recurse_to_leaf ? "_LEAF" : "",
                bucket->id, x, outpos, numrep,
                tries, recurse_tries, local_retries, local_fallback_retries,
                parent_r, stable);

        for (rep = stable ? 0 : outpos; rep < numrep && count > 0 ; rep++) {
                /* keep trying until we get a non-out, non-colliding item */
                ftotal = 0;
                skip_rep = 0;
                do {
                        retry_descent = 0;
                        in = bucket;              /* initial bucket */

                        /* choose through intervening buckets */
                        flocal = 0;
                        do {
                                collide = 0;
                                retry_bucket = 0;
                                r = rep + parent_r;
                                /* r' = r + f_total */
                                r += ftotal;

                                /* bucket choose */
                                if (in->size == 0) {
                                        reject = 1;
                                        goto reject;
                                }
                                if (local_fallback_retries > 0 &&
                                    flocal >= (in->size>>1) &&
                                    flocal > local_fallback_retries)
                                        item = bucket_perm_choose(
                                                in, work->work[-1-in->id],
                                                x, r);
                                else
                                        item = crush_bucket_choose(
                                                in, work->work[-1-in->id],
                                                x, r,
                                                (choose_args ? &choose_args[-1-in->id] : 0),
                                                outpos);
                                if (item >= map->max_devices) {
                                        dprintk("   bad item %d\n", item);
                                        skip_rep = 1;
                                        break;
                                }

                                /* desired type? */
                                if (item < 0)
                                        itemtype = map->buckets[-1-item]->type;
                                else
                                        itemtype = 0;
                                dprintk("  item %d type %d\n", item, itemtype);

                                /* keep going? */
                                if (itemtype != type) {
                                        if (item >= 0 ||
                                            (-1-item) >= map->max_buckets) {
                                                dprintk("   bad item type %d\n", type);
                                                skip_rep = 1;
                                                break;
                                        }
                                        in = map->buckets[-1-item];
                                        retry_bucket = 1;
                                        continue;
                                }

                                /* collision? */
                                for (i = 0; i < outpos; i++) {
                                        if (out[i] == item) {
                                                collide = 1;
                                                break;
                                        }
                                }

                                reject = 0;
                                if (!collide && recurse_to_leaf) {
                                        if (item < 0) {
                                                int sub_r;
                                                if (vary_r)
                                                        sub_r = r >> (vary_r-1);
                                                else
                                                        sub_r = 0;
                                                if (crush_choose_firstn(
                                                            map,
                                                            work,
                                                            map->buckets[-1-item],
                                                            weight, weight_max,
                                                            x, stable ? 1 : outpos+1, 0,
                                                            out2, outpos, count,
                                                            recurse_tries, 0,
                                                            local_retries,
                                                            local_fallback_retries,
                                                            0,
                                                            vary_r,
                                                            stable,
                                                            NULL,
                                                            sub_r,
                                                            choose_args) <= outpos)
                                                        /* didn't get leaf */
                                                        reject = 1;
                                        } else {
                                                /* we already have a leaf! */
                                                out2[outpos] = item;
                                        }
                                }

                                if (!reject && !collide) {
                                        /* out? */
                                        if (itemtype == 0)
                                                reject = is_out(map, weight,
                                                                weight_max,
                                                                item, x);
                                }

reject:
                                if (reject || collide) {
                                        ftotal++;
                                        flocal++;

                                        if (collide && flocal <= local_retries)
                                                /* retry locally a few times */
                                                retry_bucket = 1;
                                        else if (local_fallback_retries > 0 &&
                                                 flocal <= in->size + local_fallback_retries)
                                                /* exhaustive bucket search */
                                                retry_bucket = 1;
                                        else if (ftotal < tries)
                                                /* then retry descent */
                                                retry_descent = 1;
                                        else
                                                /* else give up */
                                                skip_rep = 1;
                                        dprintk("  reject %d  collide %d  "
                                                "ftotal %u  flocal %u\n",
                                                reject, collide, ftotal,
                                                flocal);
                                }
                        } while (retry_bucket);
                } while (retry_descent);

                if (skip_rep) {
                        dprintk("skip rep\n");
                        continue;
                }

                dprintk("CHOOSE got %d\n", item);
                out[outpos] = item;
                outpos++;
                count--;
#ifndef __KERNEL__
                if (map->choose_tries && ftotal <= map->choose_total_tries)
                        map->choose_tries[ftotal]++;
#endif
        }

        dprintk("CHOOSE returns %d\n", outpos);
        return outpos;
}


/**
 * crush_choose_indep: alternative breadth-first positionally stable mapping
 *
 */
static void crush_choose_indep(const struct crush_map *map,
                               struct crush_work *work,
                               const struct crush_bucket *bucket,
                               const __u32 *weight, int weight_max,
                               int x, int left, int numrep, int type,
                               int *out, int outpos,
                               unsigned int tries,
                               unsigned int recurse_tries,
                               int recurse_to_leaf,
                               int *out2,
                               int parent_r,
                               const struct crush_choose_arg *choose_args)
{
        const struct crush_bucket *in = bucket;
        int endpos = outpos + left;
        int rep;
        unsigned int ftotal;
        int r;
        int i;
        int item = 0;
        int itemtype;
        int collide;

        dprintk("CHOOSE%s INDEP bucket %d x %d outpos %d numrep %d\n", recurse_to_leaf ? "_LEAF" : "",
                bucket->id, x, outpos, numrep);

        /* initially my result is undefined */
        for (rep = outpos; rep < endpos; rep++) {
                out[rep] = CRUSH_ITEM_UNDEF;
                if (out2)
                        out2[rep] = CRUSH_ITEM_UNDEF;
        }

        for (ftotal = 0; left > 0 && ftotal < tries; ftotal++) {
#ifdef DEBUG_INDEP
                if (out2 && ftotal) {
                        dprintk("%u %d a: ", ftotal, left);
                        for (rep = outpos; rep < endpos; rep++) {
                                dprintk(" %d", out[rep]);
                        }
                        dprintk("\n");
                        dprintk("%u %d b: ", ftotal, left);
                        for (rep = outpos; rep < endpos; rep++) {
                                dprintk(" %d", out2[rep]);
                        }
                        dprintk("\n");
                }
#endif
                for (rep = outpos; rep < endpos; rep++) {
                        if (out[rep] != CRUSH_ITEM_UNDEF)
                                continue;

                        in = bucket;  /* initial bucket */

                        /* choose through intervening buckets */
                        for (;;) {
                                /* note: we base the choice on the position
                                 * even in the nested call.  that means that
                                 * if the first layer chooses the same bucket
                                 * in a different position, we will tend to
                                 * choose a different item in that bucket.
                                 * this will involve more devices in data
                                 * movement and tend to distribute the load.
                                 */
                                r = rep + parent_r;

                                /* be careful */
                                if (in->alg == CRUSH_BUCKET_UNIFORM &&
                                    in->size % numrep == 0)
                                        /* r'=r+(n+1)*f_total */
                                        r += (numrep+1) * ftotal;
                                else
                                        /* r' = r + n*f_total */
                                        r += numrep * ftotal;

                                /* bucket choose */
                                if (in->size == 0) {
                                        dprintk("   empty bucket\n");
                                        break;
                                }

                                item = crush_bucket_choose(
                                        in, work->work[-1-in->id],
                                        x, r,
                                        (choose_args ? &choose_args[-1-in->id] : 0),
                                        outpos);
                                if (item >= map->max_devices) {
                                        dprintk("   bad item %d\n", item);
                                        out[rep] = CRUSH_ITEM_NONE;
                                        if (out2)
                                                out2[rep] = CRUSH_ITEM_NONE;
                                        left--;
                                        break;
                                }

                                /* desired type? */
                                if (item < 0)
                                        itemtype = map->buckets[-1-item]->type;
                                else
                                        itemtype = 0;
                                dprintk("  item %d type %d\n", item, itemtype);

                                /* keep going? */
                                if (itemtype != type) {
                                        if (item >= 0 ||
                                            (-1-item) >= map->max_buckets) {
                                                dprintk("   bad item type %d\n", type);
                                                out[rep] = CRUSH_ITEM_NONE;
                                                if (out2)
                                                        out2[rep] =
                                                                CRUSH_ITEM_NONE;
                                                left--;
                                                break;
                                        }
                                        in = map->buckets[-1-item];
                                        continue;
                                }

                                /* collision? */
                                collide = 0;
                                for (i = outpos; i < endpos; i++) {
                                        if (out[i] == item) {
                                                collide = 1;
                                                break;
                                        }
                                }
                                if (collide)
                                        break;

                                if (recurse_to_leaf) {
                                        if (item < 0) {
                                                crush_choose_indep(
                                                        map,
                                                        work,
                                                        map->buckets[-1-item],
                                                        weight, weight_max,
                                                        x, 1, numrep, 0,
                                                        out2, rep,
                                                        recurse_tries, 0,
                                                        0, NULL, r, choose_args);
                                                if (out2[rep] == CRUSH_ITEM_NONE) {
                                                        /* placed nothing; no leaf */
                                                        break;
                                                }
                                        } else {
                                                /* we already have a leaf! */
                                                out2[rep] = item;
                                        }
                                }

                                /* out? */
                                if (itemtype == 0 &&
                                    is_out(map, weight, weight_max, item, x))
                                        break;

                                /* yay! */
                                out[rep] = item;
                                left--;
                                break;
                        }
                }
        }
        for (rep = outpos; rep < endpos; rep++) {
                if (out[rep] == CRUSH_ITEM_UNDEF) {
                        out[rep] = CRUSH_ITEM_NONE;
                }
                if (out2 && out2[rep] == CRUSH_ITEM_UNDEF) {
                        out2[rep] = CRUSH_ITEM_NONE;
                }
        }
#ifndef __KERNEL__
        if (map->choose_tries && ftotal <= map->choose_total_tries)
                map->choose_tries[ftotal]++;
#endif
#ifdef DEBUG_INDEP
        if (out2) {
                dprintk("%u %d a: ", ftotal, left);
                for (rep = outpos; rep < endpos; rep++) {
                        dprintk(" %d", out[rep]);
                }
                dprintk("\n");
                dprintk("%u %d b: ", ftotal, left);
                for (rep = outpos; rep < endpos; rep++) {
                        dprintk(" %d", out2[rep]);
                }
                dprintk("\n");
        }
#endif
}


/* This takes a chunk of memory and sets it up to be a shiny new
   working area for a CRUSH placement computation. It must be called
   on any newly allocated memory before passing it in to
   crush_do_rule. It may be used repeatedly after that, so long as the
   map has not changed. If the map /has/ changed, you must make sure
   the working size is no smaller than what was allocated and re-run
   crush_init_workspace.

   If you do retain the working space between calls to crush, make it
   thread-local. If you reinstitute the locking I've spent so much
   time getting rid of, I will be very unhappy with you. */

void crush_init_workspace(const struct crush_map *m, void *v) {
        /* We work by moving through the available space and setting
           values and pointers as we go.

           It's a bit like Forth's use of the 'allot' word since we
           set the pointer first and then reserve the space for it to
           point to by incrementing the point. */
        struct crush_work *w = (struct crush_work *)v;
        char *point = (char *)v;
        __s32 b;
        point += sizeof(struct crush_work);
        w->work = (struct crush_work_bucket **)point;
        point += m->max_buckets * sizeof(struct crush_work_bucket *);
        for (b = 0; b < m->max_buckets; ++b) {
                if (m->buckets[b] == 0)
                        continue;

                w->work[b] = (struct crush_work_bucket *) point;
                switch (m->buckets[b]->alg) {
                default:
                        point += sizeof(struct crush_work_bucket);
                        break;
                }
                w->work[b]->perm_x = 0;
                w->work[b]->perm_n = 0;
                w->work[b]->perm = (__u32 *)point;
                point += m->buckets[b]->size * sizeof(__u32);
        }
        BUG_ON((char *)point - (char *)w != m->working_size);
}

/**
 * crush_do_rule - calculate a mapping with the given input and rule
 * @map: the crush_map
 * @ruleno: the rule id
 * @x: hash input
 * @result: pointer to result vector
 * @result_max: maximum result size
 * @weight: weight vector (for map leaves)
 * @weight_max: size of weight vector
 * @cwin: Pointer to at least map->working_size bytes of memory or NULL.
 */
int crush_do_rule(const struct crush_map *map,
                  int ruleno, int x, int *result, int result_max,
                  const __u32 *weight, int weight_max,
                  void *cwin, const struct crush_choose_arg *choose_args)
{
        int result_len;
        struct crush_work *cw = cwin;
        int *a = (int *)((char *)cw + map->working_size);
        int *b = a + result_max;
        int *c = b + result_max;
        int *w = a;
        int *o = b;
        int recurse_to_leaf;
        int wsize = 0;
        int osize;
        int *tmp;
        const struct crush_rule *rule;
        __u32 step;
        int i, j;
        int numrep;
        int out_size;
        /*
         * the original choose_total_tries value was off by one (it
         * counted "retries" and not "tries").  add one.
         */
        int choose_tries = map->choose_total_tries + 1;
        int choose_leaf_tries = 0;
        /*
         * the local tries values were counted as "retries", though,
         * and need no adjustment
         */
        int choose_local_retries = map->choose_local_tries;
        int choose_local_fallback_retries = map->choose_local_fallback_tries;

        int vary_r = map->chooseleaf_vary_r;
        int stable = map->chooseleaf_stable;

        if ((__u32)ruleno >= map->max_rules) {
                dprintk(" bad ruleno %d\n", ruleno);
                return 0;
        }

        rule = map->rules[ruleno];
        result_len = 0;

        for (step = 0; step < rule->len; step++) {
                int firstn = 0;
                const struct crush_rule_step *curstep = &rule->steps[step];

                switch (curstep->op) {
                case CRUSH_RULE_TAKE:
                        if ((curstep->arg1 >= 0 &&
                             curstep->arg1 < map->max_devices) ||
                            (-1-curstep->arg1 >= 0 &&
                             -1-curstep->arg1 < map->max_buckets &&
                             map->buckets[-1-curstep->arg1])) {
                                w[0] = curstep->arg1;
                                wsize = 1;
                        } else {
                                dprintk(" bad take value %d\n", curstep->arg1);
                        }
                        break;

                case CRUSH_RULE_SET_CHOOSE_TRIES:
                        if (curstep->arg1 > 0)
                                choose_tries = curstep->arg1;
                        break;

                case CRUSH_RULE_SET_CHOOSELEAF_TRIES:
                        if (curstep->arg1 > 0)
                                choose_leaf_tries = curstep->arg1;
                        break;

                case CRUSH_RULE_SET_CHOOSE_LOCAL_TRIES:
                        if (curstep->arg1 >= 0)
                                choose_local_retries = curstep->arg1;
                        break;

                case CRUSH_RULE_SET_CHOOSE_LOCAL_FALLBACK_TRIES:
                        if (curstep->arg1 >= 0)
                                choose_local_fallback_retries = curstep->arg1;
                        break;

                case CRUSH_RULE_SET_CHOOSELEAF_VARY_R:
                        if (curstep->arg1 >= 0)
                                vary_r = curstep->arg1;
                        break;

                case CRUSH_RULE_SET_CHOOSELEAF_STABLE:
                        if (curstep->arg1 >= 0)
                                stable = curstep->arg1;
                        break;

                case CRUSH_RULE_CHOOSELEAF_FIRSTN:
                case CRUSH_RULE_CHOOSE_FIRSTN:
                        firstn = 1;
                        /* fall through */
                case CRUSH_RULE_CHOOSELEAF_INDEP:
                case CRUSH_RULE_CHOOSE_INDEP:
                        if (wsize == 0)
                                break;

                        recurse_to_leaf =
                                curstep->op ==
                                 CRUSH_RULE_CHOOSELEAF_FIRSTN ||
                                curstep->op ==
                                CRUSH_RULE_CHOOSELEAF_INDEP;

                        /* reset output */
                        osize = 0;

                        for (i = 0; i < wsize; i++) {
                                int bno;
                                numrep = curstep->arg1;
                                if (numrep <= 0) {
                                        numrep += result_max;
                                        if (numrep <= 0)
                                                continue;
                                }
                                j = 0;
                                /* make sure bucket id is valid */
                                bno = -1 - w[i];
                                if (bno < 0 || bno >= map->max_buckets) {
                                        // w[i] is probably CRUSH_ITEM_NONE
                                        dprintk("  bad w[i] %d\n", w[i]);
                                        continue;
                                }
                                if (firstn) {
                                        int recurse_tries;
                                        if (choose_leaf_tries)
                                                recurse_tries =
                                                        choose_leaf_tries;
                                        else if (map->chooseleaf_descend_once)
                                                recurse_tries = 1;
                                        else
                                                recurse_tries = choose_tries;
                                        osize += crush_choose_firstn(
                                                map,
                                                cw,
                                                map->buckets[bno],
                                                weight, weight_max,
                                                x, numrep,
                                                curstep->arg2,
                                                o+osize, j,
                                                result_max-osize,
                                                choose_tries,
                                                recurse_tries,
                                                choose_local_retries,
                                                choose_local_fallback_retries,
                                                recurse_to_leaf,
                                                vary_r,
                                                stable,
                                                c+osize,
                                                0,
                                                choose_args);
                                } else {
                                        out_size = ((numrep < (result_max-osize)) ?
                                                    numrep : (result_max-osize));
                                        crush_choose_indep(
                                                map,
                                                cw,
                                                map->buckets[bno],
                                                weight, weight_max,
                                                x, out_size, numrep,
                                                curstep->arg2,
                                                o+osize, j,
                                                choose_tries,
                                                choose_leaf_tries ?
                                                   choose_leaf_tries : 1,
                                                recurse_to_leaf,
                                                c+osize,
                                                0,
                                                choose_args);
                                        osize += out_size;
                                }
                        }

                        if (recurse_to_leaf)
                                /* copy final _leaf_ values to output set */
                                memcpy(o, c, osize*sizeof(*o));

                        /* swap o and w arrays */
                        tmp = o;
                        o = w;
                        w = tmp;
                        wsize = osize;
                        break;


                case CRUSH_RULE_EMIT:
                        for (i = 0; i < wsize && result_len < result_max; i++) {
                                result[result_len] = w[i];
                                result_len++;
                        }
                        wsize = 0;
                        break;

                default:
                        dprintk(" unknown op %d at step %d\n",
                                curstep->op, step);
                        break;
                }
        }

        return result_len;
}