include cleanup: Update gfp.h and slab.h includes to prepare for breaking implicit...

[mirror_ubuntu-artful-kernel.git] / fs / ceph / snap.c

#include "ceph_debug.h"

#include <linux/sort.h>
#include <linux/slab.h>

#include "super.h"
#include "decode.h"

/*
 * Snapshots in ceph are driven in large part by cooperation from the
 * client.  In contrast to local file systems or file servers that
 * implement snapshots at a single point in the system, ceph's
 * distributed access to storage requires clients to help decide
 * whether a write logically occurs before or after a recently created
 * snapshot.
 *
 * This provides a perfect instantanous client-wide snapshot.  Between
 * clients, however, snapshots may appear to be applied at slightly
 * different points in time, depending on delays in delivering the
 * snapshot notification.
 *
 * Snapshots are _not_ file system-wide.  Instead, each snapshot
 * applies to the subdirectory nested beneath some directory.  This
 * effectively divides the hierarchy into multiple "realms," where all
 * of the files contained by each realm share the same set of
 * snapshots.  An individual realm's snap set contains snapshots
 * explicitly created on that realm, as well as any snaps in its
 * parent's snap set _after_ the point at which the parent became it's
 * parent (due to, say, a rename).  Similarly, snaps from prior parents
 * during the time intervals during which they were the parent are included.
 *
 * The client is spared most of this detail, fortunately... it must only
 * maintains a hierarchy of realms reflecting the current parent/child
 * realm relationship, and for each realm has an explicit list of snaps
 * inherited from prior parents.
 *
 * A snap_realm struct is maintained for realms containing every inode
 * with an open cap in the system.  (The needed snap realm information is
 * provided by the MDS whenever a cap is issued, i.e., on open.)  A 'seq'
 * version number is used to ensure that as realm parameters change (new
 * snapshot, new parent, etc.) the client's realm hierarchy is updated.
 *
 * The realm hierarchy drives the generation of a 'snap context' for each
 * realm, which simply lists the resulting set of snaps for the realm.  This
 * is attached to any writes sent to OSDs.
 */
/*
 * Unfortunately error handling is a bit mixed here.  If we get a snap
 * update, but don't have enough memory to update our realm hierarchy,
 * it's not clear what we can do about it (besides complaining to the
 * console).
 */


/*
 * increase ref count for the realm
 *
 * caller must hold snap_rwsem for write.
 */
void ceph_get_snap_realm(struct ceph_mds_client *mdsc,
                         struct ceph_snap_realm *realm)
{
        dout("get_realm %p %d -> %d\n", realm,
             atomic_read(&realm->nref), atomic_read(&realm->nref)+1);
        /*
         * since we _only_ increment realm refs or empty the empty
         * list with snap_rwsem held, adjusting the empty list here is
         * safe.  we do need to protect against concurrent empty list
         * additions, however.
         */
        if (atomic_read(&realm->nref) == 0) {
                spin_lock(&mdsc->snap_empty_lock);
                list_del_init(&realm->empty_item);
                spin_unlock(&mdsc->snap_empty_lock);
        }

        atomic_inc(&realm->nref);
}

static void __insert_snap_realm(struct rb_root *root,
                                struct ceph_snap_realm *new)
{
        struct rb_node **p = &root->rb_node;
        struct rb_node *parent = NULL;
        struct ceph_snap_realm *r = NULL;

        while (*p) {
                parent = *p;
                r = rb_entry(parent, struct ceph_snap_realm, node);
                if (new->ino < r->ino)
                        p = &(*p)->rb_left;
                else if (new->ino > r->ino)
                        p = &(*p)->rb_right;
                else
                        BUG();
        }

        rb_link_node(&new->node, parent, p);
        rb_insert_color(&new->node, root);
}

/*
 * create and get the realm rooted at @ino and bump its ref count.
 *
 * caller must hold snap_rwsem for write.
 */
static struct ceph_snap_realm *ceph_create_snap_realm(
        struct ceph_mds_client *mdsc,
        u64 ino)
{
        struct ceph_snap_realm *realm;

        realm = kzalloc(sizeof(*realm), GFP_NOFS);
        if (!realm)
                return ERR_PTR(-ENOMEM);

        atomic_set(&realm->nref, 0);    /* tree does not take a ref */
        realm->ino = ino;
        INIT_LIST_HEAD(&realm->children);
        INIT_LIST_HEAD(&realm->child_item);
        INIT_LIST_HEAD(&realm->empty_item);
        INIT_LIST_HEAD(&realm->inodes_with_caps);
        spin_lock_init(&realm->inodes_with_caps_lock);
        __insert_snap_realm(&mdsc->snap_realms, realm);
        dout("create_snap_realm %llx %p\n", realm->ino, realm);
        return realm;
}

/*
 * lookup the realm rooted at @ino.
 *
 * caller must hold snap_rwsem for write.
 */
struct ceph_snap_realm *ceph_lookup_snap_realm(struct ceph_mds_client *mdsc,
                                               u64 ino)
{
        struct rb_node *n = mdsc->snap_realms.rb_node;
        struct ceph_snap_realm *r;

        while (n) {
                r = rb_entry(n, struct ceph_snap_realm, node);
                if (ino < r->ino)
                        n = n->rb_left;
                else if (ino > r->ino)
                        n = n->rb_right;
                else {
                        dout("lookup_snap_realm %llx %p\n", r->ino, r);
                        return r;
                }
        }
        return NULL;
}

static void __put_snap_realm(struct ceph_mds_client *mdsc,
                             struct ceph_snap_realm *realm);

/*
 * called with snap_rwsem (write)
 */
static void __destroy_snap_realm(struct ceph_mds_client *mdsc,
                                 struct ceph_snap_realm *realm)
{
        dout("__destroy_snap_realm %p %llx\n", realm, realm->ino);

        rb_erase(&realm->node, &mdsc->snap_realms);

        if (realm->parent) {
                list_del_init(&realm->child_item);
                __put_snap_realm(mdsc, realm->parent);
        }

        kfree(realm->prior_parent_snaps);
        kfree(realm->snaps);
        ceph_put_snap_context(realm->cached_context);
        kfree(realm);
}

/*
 * caller holds snap_rwsem (write)
 */
static void __put_snap_realm(struct ceph_mds_client *mdsc,
                             struct ceph_snap_realm *realm)
{
        dout("__put_snap_realm %llx %p %d -> %d\n", realm->ino, realm,
             atomic_read(&realm->nref), atomic_read(&realm->nref)-1);
        if (atomic_dec_and_test(&realm->nref))
                __destroy_snap_realm(mdsc, realm);
}

/*
 * caller needn't hold any locks
 */
void ceph_put_snap_realm(struct ceph_mds_client *mdsc,
                         struct ceph_snap_realm *realm)
{
        dout("put_snap_realm %llx %p %d -> %d\n", realm->ino, realm,
             atomic_read(&realm->nref), atomic_read(&realm->nref)-1);
        if (!atomic_dec_and_test(&realm->nref))
                return;

        if (down_write_trylock(&mdsc->snap_rwsem)) {
                __destroy_snap_realm(mdsc, realm);
                up_write(&mdsc->snap_rwsem);
        } else {
                spin_lock(&mdsc->snap_empty_lock);
                list_add(&mdsc->snap_empty, &realm->empty_item);
                spin_unlock(&mdsc->snap_empty_lock);
        }
}

/*
 * Clean up any realms whose ref counts have dropped to zero.  Note
 * that this does not include realms who were created but not yet
 * used.
 *
 * Called under snap_rwsem (write)
 */
static void __cleanup_empty_realms(struct ceph_mds_client *mdsc)
{
        struct ceph_snap_realm *realm;

        spin_lock(&mdsc->snap_empty_lock);
        while (!list_empty(&mdsc->snap_empty)) {
                realm = list_first_entry(&mdsc->snap_empty,
                                   struct ceph_snap_realm, empty_item);
                list_del(&realm->empty_item);
                spin_unlock(&mdsc->snap_empty_lock);
                __destroy_snap_realm(mdsc, realm);
                spin_lock(&mdsc->snap_empty_lock);
        }
        spin_unlock(&mdsc->snap_empty_lock);
}

void ceph_cleanup_empty_realms(struct ceph_mds_client *mdsc)
{
        down_write(&mdsc->snap_rwsem);
        __cleanup_empty_realms(mdsc);
        up_write(&mdsc->snap_rwsem);
}

/*
 * adjust the parent realm of a given @realm.  adjust child list, and parent
 * pointers, and ref counts appropriately.
 *
 * return true if parent was changed, 0 if unchanged, <0 on error.
 *
 * caller must hold snap_rwsem for write.
 */
static int adjust_snap_realm_parent(struct ceph_mds_client *mdsc,
                                    struct ceph_snap_realm *realm,
                                    u64 parentino)
{
        struct ceph_snap_realm *parent;

        if (realm->parent_ino == parentino)
                return 0;

        parent = ceph_lookup_snap_realm(mdsc, parentino);
        if (!parent) {
                parent = ceph_create_snap_realm(mdsc, parentino);
                if (IS_ERR(parent))
                        return PTR_ERR(parent);
        }
        dout("adjust_snap_realm_parent %llx %p: %llx %p -> %llx %p\n",
             realm->ino, realm, realm->parent_ino, realm->parent,
             parentino, parent);
        if (realm->parent) {
                list_del_init(&realm->child_item);
                ceph_put_snap_realm(mdsc, realm->parent);
        }
        realm->parent_ino = parentino;
        realm->parent = parent;
        ceph_get_snap_realm(mdsc, parent);
        list_add(&realm->child_item, &parent->children);
        return 1;
}


static int cmpu64_rev(const void *a, const void *b)
{
        if (*(u64 *)a < *(u64 *)b)
                return 1;
        if (*(u64 *)a > *(u64 *)b)
                return -1;
        return 0;
}

/*
 * build the snap context for a given realm.
 */
static int build_snap_context(struct ceph_snap_realm *realm)
{
        struct ceph_snap_realm *parent = realm->parent;
        struct ceph_snap_context *snapc;
        int err = 0;
        int i;
        int num = realm->num_prior_parent_snaps + realm->num_snaps;

        /*
         * build parent context, if it hasn't been built.
         * conservatively estimate that all parent snaps might be
         * included by us.
         */
        if (parent) {
                if (!parent->cached_context) {
                        err = build_snap_context(parent);
                        if (err)
                                goto fail;
                }
                num += parent->cached_context->num_snaps;
        }

        /* do i actually need to update?  not if my context seq
           matches realm seq, and my parents' does to.  (this works
           because we rebuild_snap_realms() works _downward_ in
           hierarchy after each update.) */
        if (realm->cached_context &&
            realm->cached_context->seq == realm->seq &&
            (!parent ||
             realm->cached_context->seq >= parent->cached_context->seq)) {
                dout("build_snap_context %llx %p: %p seq %lld (%d snaps)"
                     " (unchanged)\n",
                     realm->ino, realm, realm->cached_context,
                     realm->cached_context->seq,
                     realm->cached_context->num_snaps);
                return 0;
        }

        /* alloc new snap context */
        err = -ENOMEM;
        if (num > ULONG_MAX / sizeof(u64) - sizeof(*snapc))
                goto fail;
        snapc = kzalloc(sizeof(*snapc) + num*sizeof(u64), GFP_NOFS);
        if (!snapc)
                goto fail;
        atomic_set(&snapc->nref, 1);

        /* build (reverse sorted) snap vector */
        num = 0;
        snapc->seq = realm->seq;
        if (parent) {
                /* include any of parent's snaps occuring _after_ my
                   parent became my parent */
                for (i = 0; i < parent->cached_context->num_snaps; i++)
                        if (parent->cached_context->snaps[i] >=
                            realm->parent_since)
                                snapc->snaps[num++] =
                                        parent->cached_context->snaps[i];
                if (parent->cached_context->seq > snapc->seq)
                        snapc->seq = parent->cached_context->seq;
        }
        memcpy(snapc->snaps + num, realm->snaps,
               sizeof(u64)*realm->num_snaps);
        num += realm->num_snaps;
        memcpy(snapc->snaps + num, realm->prior_parent_snaps,
               sizeof(u64)*realm->num_prior_parent_snaps);
        num += realm->num_prior_parent_snaps;

        sort(snapc->snaps, num, sizeof(u64), cmpu64_rev, NULL);
        snapc->num_snaps = num;
        dout("build_snap_context %llx %p: %p seq %lld (%d snaps)\n",
             realm->ino, realm, snapc, snapc->seq, snapc->num_snaps);

        if (realm->cached_context)
                ceph_put_snap_context(realm->cached_context);
        realm->cached_context = snapc;
        return 0;

fail:
        /*
         * if we fail, clear old (incorrect) cached_context... hopefully
         * we'll have better luck building it later
         */
        if (realm->cached_context) {
                ceph_put_snap_context(realm->cached_context);
                realm->cached_context = NULL;
        }
        pr_err("build_snap_context %llx %p fail %d\n", realm->ino,
               realm, err);
        return err;
}

/*
 * rebuild snap context for the given realm and all of its children.
 */
static void rebuild_snap_realms(struct ceph_snap_realm *realm)
{
        struct ceph_snap_realm *child;

        dout("rebuild_snap_realms %llx %p\n", realm->ino, realm);
        build_snap_context(realm);

        list_for_each_entry(child, &realm->children, child_item)
                rebuild_snap_realms(child);
}


/*
 * helper to allocate and decode an array of snapids.  free prior
 * instance, if any.
 */
static int dup_array(u64 **dst, __le64 *src, int num)
{
        int i;

        kfree(*dst);
        if (num) {
                *dst = kcalloc(num, sizeof(u64), GFP_NOFS);
                if (!*dst)
                        return -ENOMEM;
                for (i = 0; i < num; i++)
                        (*dst)[i] = get_unaligned_le64(src + i);
        } else {
                *dst = NULL;
        }
        return 0;
}


/*
 * When a snapshot is applied, the size/mtime inode metadata is queued
 * in a ceph_cap_snap (one for each snapshot) until writeback
 * completes and the metadata can be flushed back to the MDS.
 *
 * However, if a (sync) write is currently in-progress when we apply
 * the snapshot, we have to wait until the write succeeds or fails
 * (and a final size/mtime is known).  In this case the
 * cap_snap->writing = 1, and is said to be "pending."  When the write
 * finishes, we __ceph_finish_cap_snap().
 *
 * Caller must hold snap_rwsem for read (i.e., the realm topology won't
 * change).
 */
void ceph_queue_cap_snap(struct ceph_inode_info *ci,
                         struct ceph_snap_context *snapc)
{
        struct inode *inode = &ci->vfs_inode;
        struct ceph_cap_snap *capsnap;
        int used;

        capsnap = kzalloc(sizeof(*capsnap), GFP_NOFS);
        if (!capsnap) {
                pr_err("ENOMEM allocating ceph_cap_snap on %p\n", inode);
                return;
        }

        spin_lock(&inode->i_lock);
        used = __ceph_caps_used(ci);
        if (__ceph_have_pending_cap_snap(ci)) {
                /* there is no point in queuing multiple "pending" cap_snaps,
                   as no new writes are allowed to start when pending, so any
                   writes in progress now were started before the previous
                   cap_snap.  lucky us. */
                dout("queue_cap_snap %p snapc %p seq %llu used %d"
                     " already pending\n", inode, snapc, snapc->seq, used);
                kfree(capsnap);
        } else if (ci->i_wrbuffer_ref_head || (used & CEPH_CAP_FILE_WR)) {
                igrab(inode);

                atomic_set(&capsnap->nref, 1);
                capsnap->ci = ci;
                INIT_LIST_HEAD(&capsnap->ci_item);
                INIT_LIST_HEAD(&capsnap->flushing_item);

                capsnap->follows = snapc->seq - 1;
                capsnap->context = ceph_get_snap_context(snapc);
                capsnap->issued = __ceph_caps_issued(ci, NULL);
                capsnap->dirty = __ceph_caps_dirty(ci);

                capsnap->mode = inode->i_mode;
                capsnap->uid = inode->i_uid;
                capsnap->gid = inode->i_gid;

                /* fixme? */
                capsnap->xattr_blob = NULL;
                capsnap->xattr_len = 0;

                /* dirty page count moved from _head to this cap_snap;
                   all subsequent writes page dirties occur _after_ this
                   snapshot. */
                capsnap->dirty_pages = ci->i_wrbuffer_ref_head;
                ci->i_wrbuffer_ref_head = 0;
                ceph_put_snap_context(ci->i_head_snapc);
                ci->i_head_snapc = NULL;
                list_add_tail(&capsnap->ci_item, &ci->i_cap_snaps);

                if (used & CEPH_CAP_FILE_WR) {
                        dout("queue_cap_snap %p cap_snap %p snapc %p"
                             " seq %llu used WR, now pending\n", inode,
                             capsnap, snapc, snapc->seq);
                        capsnap->writing = 1;
                } else {
                        /* note mtime, size NOW. */
                        __ceph_finish_cap_snap(ci, capsnap);
                }
        } else {
                dout("queue_cap_snap %p nothing dirty|writing\n", inode);
                kfree(capsnap);
        }

        spin_unlock(&inode->i_lock);
}

/*
 * Finalize the size, mtime for a cap_snap.. that is, settle on final values
 * to be used for the snapshot, to be flushed back to the mds.
 *
 * If capsnap can now be flushed, add to snap_flush list, and return 1.
 *
 * Caller must hold i_lock.
 */
int __ceph_finish_cap_snap(struct ceph_inode_info *ci,
                            struct ceph_cap_snap *capsnap)
{
        struct inode *inode = &ci->vfs_inode;
        struct ceph_mds_client *mdsc = &ceph_client(inode->i_sb)->mdsc;

        BUG_ON(capsnap->writing);
        capsnap->size = inode->i_size;
        capsnap->mtime = inode->i_mtime;
        capsnap->atime = inode->i_atime;
        capsnap->ctime = inode->i_ctime;
        capsnap->time_warp_seq = ci->i_time_warp_seq;
        if (capsnap->dirty_pages) {
                dout("finish_cap_snap %p cap_snap %p snapc %p %llu s=%llu "
                     "still has %d dirty pages\n", inode, capsnap,
                     capsnap->context, capsnap->context->seq,
                     capsnap->size, capsnap->dirty_pages);
                return 0;
        }
        dout("finish_cap_snap %p cap_snap %p snapc %p %llu s=%llu clean\n",
             inode, capsnap, capsnap->context,
             capsnap->context->seq, capsnap->size);

        spin_lock(&mdsc->snap_flush_lock);
        list_add_tail(&ci->i_snap_flush_item, &mdsc->snap_flush_list);
        spin_unlock(&mdsc->snap_flush_lock);
        return 1;  /* caller may want to ceph_flush_snaps */
}


/*
 * Parse and apply a snapblob "snap trace" from the MDS.  This specifies
 * the snap realm parameters from a given realm and all of its ancestors,
 * up to the root.
 *
 * Caller must hold snap_rwsem for write.
 */
int ceph_update_snap_trace(struct ceph_mds_client *mdsc,
                           void *p, void *e, bool deletion)
{
        struct ceph_mds_snap_realm *ri;    /* encoded */
        __le64 *snaps;                     /* encoded */
        __le64 *prior_parent_snaps;        /* encoded */
        struct ceph_snap_realm *realm;
        int invalidate = 0;
        int err = -ENOMEM;

        dout("update_snap_trace deletion=%d\n", deletion);
more:
        ceph_decode_need(&p, e, sizeof(*ri), bad);
        ri = p;
        p += sizeof(*ri);
        ceph_decode_need(&p, e, sizeof(u64)*(le32_to_cpu(ri->num_snaps) +
                            le32_to_cpu(ri->num_prior_parent_snaps)), bad);
        snaps = p;
        p += sizeof(u64) * le32_to_cpu(ri->num_snaps);
        prior_parent_snaps = p;
        p += sizeof(u64) * le32_to_cpu(ri->num_prior_parent_snaps);

        realm = ceph_lookup_snap_realm(mdsc, le64_to_cpu(ri->ino));
        if (!realm) {
                realm = ceph_create_snap_realm(mdsc, le64_to_cpu(ri->ino));
                if (IS_ERR(realm)) {
                        err = PTR_ERR(realm);
                        goto fail;
                }
        }

        if (le64_to_cpu(ri->seq) > realm->seq) {
                dout("update_snap_trace updating %llx %p %lld -> %lld\n",
                     realm->ino, realm, realm->seq, le64_to_cpu(ri->seq));
                /*
                 * if the realm seq has changed, queue a cap_snap for every
                 * inode with open caps.  we do this _before_ we update
                 * the realm info so that we prepare for writeback under the
                 * _previous_ snap context.
                 *
                 * ...unless it's a snap deletion!
                 */
                if (!deletion) {
                        struct ceph_inode_info *ci;
                        struct inode *lastinode = NULL;

                        spin_lock(&realm->inodes_with_caps_lock);
                        list_for_each_entry(ci, &realm->inodes_with_caps,
                                            i_snap_realm_item) {
                                struct inode *inode = igrab(&ci->vfs_inode);
                                if (!inode)
                                        continue;
                                spin_unlock(&realm->inodes_with_caps_lock);
                                if (lastinode)
                                        iput(lastinode);
                                lastinode = inode;
                                ceph_queue_cap_snap(ci, realm->cached_context);
                                spin_lock(&realm->inodes_with_caps_lock);
                        }
                        spin_unlock(&realm->inodes_with_caps_lock);
                        if (lastinode)
                                iput(lastinode);
                        dout("update_snap_trace cap_snaps queued\n");
                }

        } else {
                dout("update_snap_trace %llx %p seq %lld unchanged\n",
                     realm->ino, realm, realm->seq);
        }

        /* ensure the parent is correct */
        err = adjust_snap_realm_parent(mdsc, realm, le64_to_cpu(ri->parent));
        if (err < 0)
                goto fail;
        invalidate += err;

        if (le64_to_cpu(ri->seq) > realm->seq) {
                /* update realm parameters, snap lists */
                realm->seq = le64_to_cpu(ri->seq);
                realm->created = le64_to_cpu(ri->created);
                realm->parent_since = le64_to_cpu(ri->parent_since);

                realm->num_snaps = le32_to_cpu(ri->num_snaps);
                err = dup_array(&realm->snaps, snaps, realm->num_snaps);
                if (err < 0)
                        goto fail;

                realm->num_prior_parent_snaps =
                        le32_to_cpu(ri->num_prior_parent_snaps);
                err = dup_array(&realm->prior_parent_snaps, prior_parent_snaps,
                                realm->num_prior_parent_snaps);
                if (err < 0)
                        goto fail;

                invalidate = 1;
        } else if (!realm->cached_context) {
                invalidate = 1;
        }

        dout("done with %llx %p, invalidated=%d, %p %p\n", realm->ino,
             realm, invalidate, p, e);

        if (p < e)
                goto more;

        /* invalidate when we reach the _end_ (root) of the trace */
        if (invalidate)
                rebuild_snap_realms(realm);

        __cleanup_empty_realms(mdsc);
        return 0;

bad:
        err = -EINVAL;
fail:
        pr_err("update_snap_trace error %d\n", err);
        return err;
}


/*
 * Send any cap_snaps that are queued for flush.  Try to carry
 * s_mutex across multiple snap flushes to avoid locking overhead.
 *
 * Caller holds no locks.
 */
static void flush_snaps(struct ceph_mds_client *mdsc)
{
        struct ceph_inode_info *ci;
        struct inode *inode;
        struct ceph_mds_session *session = NULL;

        dout("flush_snaps\n");
        spin_lock(&mdsc->snap_flush_lock);
        while (!list_empty(&mdsc->snap_flush_list)) {
                ci = list_first_entry(&mdsc->snap_flush_list,
                                struct ceph_inode_info, i_snap_flush_item);
                inode = &ci->vfs_inode;
                igrab(inode);
                spin_unlock(&mdsc->snap_flush_lock);
                spin_lock(&inode->i_lock);
                __ceph_flush_snaps(ci, &session);
                spin_unlock(&inode->i_lock);
                iput(inode);
                spin_lock(&mdsc->snap_flush_lock);
        }
        spin_unlock(&mdsc->snap_flush_lock);

        if (session) {
                mutex_unlock(&session->s_mutex);
                ceph_put_mds_session(session);
        }
        dout("flush_snaps done\n");
}


/*
 * Handle a snap notification from the MDS.
 *
 * This can take two basic forms: the simplest is just a snap creation
 * or deletion notification on an existing realm.  This should update the
 * realm and its children.
 *
 * The more difficult case is realm creation, due to snap creation at a
 * new point in the file hierarchy, or due to a rename that moves a file or
 * directory into another realm.
 */
void ceph_handle_snap(struct ceph_mds_client *mdsc,
                      struct ceph_mds_session *session,
                      struct ceph_msg *msg)
{
        struct super_block *sb = mdsc->client->sb;
        int mds = session->s_mds;
        u64 split;
        int op;
        int trace_len;
        struct ceph_snap_realm *realm = NULL;
        void *p = msg->front.iov_base;
        void *e = p + msg->front.iov_len;
        struct ceph_mds_snap_head *h;
        int num_split_inos, num_split_realms;
        __le64 *split_inos = NULL, *split_realms = NULL;
        int i;
        int locked_rwsem = 0;

        /* decode */
        if (msg->front.iov_len < sizeof(*h))
                goto bad;
        h = p;
        op = le32_to_cpu(h->op);
        split = le64_to_cpu(h->split);   /* non-zero if we are splitting an
                                          * existing realm */
        num_split_inos = le32_to_cpu(h->num_split_inos);
        num_split_realms = le32_to_cpu(h->num_split_realms);
        trace_len = le32_to_cpu(h->trace_len);
        p += sizeof(*h);

        dout("handle_snap from mds%d op %s split %llx tracelen %d\n", mds,
             ceph_snap_op_name(op), split, trace_len);

        mutex_lock(&session->s_mutex);
        session->s_seq++;
        mutex_unlock(&session->s_mutex);

        down_write(&mdsc->snap_rwsem);
        locked_rwsem = 1;

        if (op == CEPH_SNAP_OP_SPLIT) {
                struct ceph_mds_snap_realm *ri;

                /*
                 * A "split" breaks part of an existing realm off into
                 * a new realm.  The MDS provides a list of inodes
                 * (with caps) and child realms that belong to the new
                 * child.
                 */
                split_inos = p;
                p += sizeof(u64) * num_split_inos;
                split_realms = p;
                p += sizeof(u64) * num_split_realms;
                ceph_decode_need(&p, e, sizeof(*ri), bad);
                /* we will peek at realm info here, but will _not_
                 * advance p, as the realm update will occur below in
                 * ceph_update_snap_trace. */
                ri = p;

                realm = ceph_lookup_snap_realm(mdsc, split);
                if (!realm) {
                        realm = ceph_create_snap_realm(mdsc, split);
                        if (IS_ERR(realm))
                                goto out;
                }
                ceph_get_snap_realm(mdsc, realm);

                dout("splitting snap_realm %llx %p\n", realm->ino, realm);
                for (i = 0; i < num_split_inos; i++) {
                        struct ceph_vino vino = {
                                .ino = le64_to_cpu(split_inos[i]),
                                .snap = CEPH_NOSNAP,
                        };
                        struct inode *inode = ceph_find_inode(sb, vino);
                        struct ceph_inode_info *ci;

                        if (!inode)
                                continue;
                        ci = ceph_inode(inode);

                        spin_lock(&inode->i_lock);
                        if (!ci->i_snap_realm)
                                goto skip_inode;
                        /*
                         * If this inode belongs to a realm that was
                         * created after our new realm, we experienced
                         * a race (due to another split notifications
                         * arriving from a different MDS).  So skip
                         * this inode.
                         */
                        if (ci->i_snap_realm->created >
                            le64_to_cpu(ri->created)) {
                                dout(" leaving %p in newer realm %llx %p\n",
                                     inode, ci->i_snap_realm->ino,
                                     ci->i_snap_realm);
                                goto skip_inode;
                        }
                        dout(" will move %p to split realm %llx %p\n",
                             inode, realm->ino, realm);
                        /*
                         * Remove the inode from the realm's inode
                         * list, but don't add it to the new realm
                         * yet.  We don't want the cap_snap to be
                         * queued (again) by ceph_update_snap_trace()
                         * below.  Queue it _now_, under the old context.
                         */
                        spin_lock(&realm->inodes_with_caps_lock);
                        list_del_init(&ci->i_snap_realm_item);
                        spin_unlock(&realm->inodes_with_caps_lock);
                        spin_unlock(&inode->i_lock);

                        ceph_queue_cap_snap(ci,
                                            ci->i_snap_realm->cached_context);

                        iput(inode);
                        continue;

skip_inode:
                        spin_unlock(&inode->i_lock);
                        iput(inode);
                }

                /* we may have taken some of the old realm's children. */
                for (i = 0; i < num_split_realms; i++) {
                        struct ceph_snap_realm *child =
                                ceph_lookup_snap_realm(mdsc,
                                           le64_to_cpu(split_realms[i]));
                        if (!child)
                                continue;
                        adjust_snap_realm_parent(mdsc, child, realm->ino);
                }
        }

        /*
         * update using the provided snap trace. if we are deleting a
         * snap, we can avoid queueing cap_snaps.
         */
        ceph_update_snap_trace(mdsc, p, e,
                               op == CEPH_SNAP_OP_DESTROY);

        if (op == CEPH_SNAP_OP_SPLIT) {
                /*
                 * ok, _now_ add the inodes into the new realm.
                 */
                for (i = 0; i < num_split_inos; i++) {
                        struct ceph_vino vino = {
                                .ino = le64_to_cpu(split_inos[i]),
                                .snap = CEPH_NOSNAP,
                        };
                        struct inode *inode = ceph_find_inode(sb, vino);
                        struct ceph_inode_info *ci;

                        if (!inode)
                                continue;
                        ci = ceph_inode(inode);
                        spin_lock(&inode->i_lock);
                        if (!ci->i_snap_realm)
                                goto split_skip_inode;
                        ceph_put_snap_realm(mdsc, ci->i_snap_realm);
                        spin_lock(&realm->inodes_with_caps_lock);
                        list_add(&ci->i_snap_realm_item,
                                 &realm->inodes_with_caps);
                        ci->i_snap_realm = realm;
                        spin_unlock(&realm->inodes_with_caps_lock);
                        ceph_get_snap_realm(mdsc, realm);
split_skip_inode:
                        spin_unlock(&inode->i_lock);
                        iput(inode);
                }

                /* we took a reference when we created the realm, above */
                ceph_put_snap_realm(mdsc, realm);
        }

        __cleanup_empty_realms(mdsc);

        up_write(&mdsc->snap_rwsem);

        flush_snaps(mdsc);
        return;

bad:
        pr_err("corrupt snap message from mds%d\n", mds);
        ceph_msg_dump(msg);
out:
        if (locked_rwsem)
                up_write(&mdsc->snap_rwsem);
        return;
}