UBUNTU: Ubuntu-4.13.0-45.50

[mirror_ubuntu-artful-kernel.git] / fs / ext4 / fsmap.c

/*
 * Copyright (C) 2017 Oracle.  All Rights Reserved.
 *
 * Author: Darrick J. Wong <darrick.wong@oracle.com>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it would be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA.
 */
#include "ext4.h"
#include <linux/fsmap.h>
#include "fsmap.h"
#include "mballoc.h"
#include <linux/sort.h>
#include <linux/list_sort.h>
#include <trace/events/ext4.h>

/* Convert an ext4_fsmap to an fsmap. */
void ext4_fsmap_from_internal(struct super_block *sb, struct fsmap *dest,
                              struct ext4_fsmap *src)
{
        dest->fmr_device = src->fmr_device;
        dest->fmr_flags = src->fmr_flags;
        dest->fmr_physical = src->fmr_physical << sb->s_blocksize_bits;
        dest->fmr_owner = src->fmr_owner;
        dest->fmr_offset = 0;
        dest->fmr_length = src->fmr_length << sb->s_blocksize_bits;
        dest->fmr_reserved[0] = 0;
        dest->fmr_reserved[1] = 0;
        dest->fmr_reserved[2] = 0;
}

/* Convert an fsmap to an ext4_fsmap. */
void ext4_fsmap_to_internal(struct super_block *sb, struct ext4_fsmap *dest,
                            struct fsmap *src)
{
        dest->fmr_device = src->fmr_device;
        dest->fmr_flags = src->fmr_flags;
        dest->fmr_physical = src->fmr_physical >> sb->s_blocksize_bits;
        dest->fmr_owner = src->fmr_owner;
        dest->fmr_length = src->fmr_length >> sb->s_blocksize_bits;
}

/* getfsmap query state */
struct ext4_getfsmap_info {
        struct ext4_fsmap_head  *gfi_head;
        ext4_fsmap_format_t     gfi_formatter;  /* formatting fn */
        void                    *gfi_format_arg;/* format buffer */
        ext4_fsblk_t            gfi_next_fsblk; /* next fsblock we expect */
        u32                     gfi_dev;        /* device id */
        ext4_group_t            gfi_agno;       /* bg number, if applicable */
        struct ext4_fsmap       gfi_low;        /* low rmap key */
        struct ext4_fsmap       gfi_high;       /* high rmap key */
        struct ext4_fsmap       gfi_lastfree;   /* free ext at end of last bg */
        struct list_head        gfi_meta_list;  /* fixed metadata list */
        bool                    gfi_last;       /* last extent? */
};

/* Associate a device with a getfsmap handler. */
struct ext4_getfsmap_dev {
        int                     (*gfd_fn)(struct super_block *sb,
                                      struct ext4_fsmap *keys,
                                      struct ext4_getfsmap_info *info);
        u32                     gfd_dev;
};

/* Compare two getfsmap device handlers. */
static int ext4_getfsmap_dev_compare(const void *p1, const void *p2)
{
        const struct ext4_getfsmap_dev *d1 = p1;
        const struct ext4_getfsmap_dev *d2 = p2;

        return d1->gfd_dev - d2->gfd_dev;
}

/* Compare a record against our starting point */
static bool ext4_getfsmap_rec_before_low_key(struct ext4_getfsmap_info *info,
                                             struct ext4_fsmap *rec)
{
        return rec->fmr_physical < info->gfi_low.fmr_physical;
}

/*
 * Format a reverse mapping for getfsmap, having translated rm_startblock
 * into the appropriate daddr units.
 */
static int ext4_getfsmap_helper(struct super_block *sb,
                                struct ext4_getfsmap_info *info,
                                struct ext4_fsmap *rec)
{
        struct ext4_fsmap fmr;
        struct ext4_sb_info *sbi = EXT4_SB(sb);
        ext4_fsblk_t rec_fsblk = rec->fmr_physical;
        ext4_group_t agno;
        ext4_grpblk_t cno;
        int error;

        if (fatal_signal_pending(current))
                return -EINTR;

        /*
         * Filter out records that start before our startpoint, if the
         * caller requested that.
         */
        if (ext4_getfsmap_rec_before_low_key(info, rec)) {
                rec_fsblk += rec->fmr_length;
                if (info->gfi_next_fsblk < rec_fsblk)
                        info->gfi_next_fsblk = rec_fsblk;
                return EXT4_QUERY_RANGE_CONTINUE;
        }

        /* Are we just counting mappings? */
        if (info->gfi_head->fmh_count == 0) {
                if (rec_fsblk > info->gfi_next_fsblk)
                        info->gfi_head->fmh_entries++;

                if (info->gfi_last)
                        return EXT4_QUERY_RANGE_CONTINUE;

                info->gfi_head->fmh_entries++;

                rec_fsblk += rec->fmr_length;
                if (info->gfi_next_fsblk < rec_fsblk)
                        info->gfi_next_fsblk = rec_fsblk;
                return EXT4_QUERY_RANGE_CONTINUE;
        }

        /*
         * If the record starts past the last physical block we saw,
         * then we've found a gap.  Report the gap as being owned by
         * whatever the caller specified is the missing owner.
         */
        if (rec_fsblk > info->gfi_next_fsblk) {
                if (info->gfi_head->fmh_entries >= info->gfi_head->fmh_count)
                        return EXT4_QUERY_RANGE_ABORT;

                ext4_get_group_no_and_offset(sb, info->gfi_next_fsblk,
                                &agno, &cno);
                trace_ext4_fsmap_mapping(sb, info->gfi_dev, agno,
                                EXT4_C2B(sbi, cno),
                                rec_fsblk - info->gfi_next_fsblk,
                                EXT4_FMR_OWN_UNKNOWN);

                fmr.fmr_device = info->gfi_dev;
                fmr.fmr_physical = info->gfi_next_fsblk;
                fmr.fmr_owner = EXT4_FMR_OWN_UNKNOWN;
                fmr.fmr_length = rec_fsblk - info->gfi_next_fsblk;
                fmr.fmr_flags = FMR_OF_SPECIAL_OWNER;
                error = info->gfi_formatter(&fmr, info->gfi_format_arg);
                if (error)
                        return error;
                info->gfi_head->fmh_entries++;
        }

        if (info->gfi_last)
                goto out;

        /* Fill out the extent we found */
        if (info->gfi_head->fmh_entries >= info->gfi_head->fmh_count)
                return EXT4_QUERY_RANGE_ABORT;

        ext4_get_group_no_and_offset(sb, rec_fsblk, &agno, &cno);
        trace_ext4_fsmap_mapping(sb, info->gfi_dev, agno, EXT4_C2B(sbi, cno),
                        rec->fmr_length, rec->fmr_owner);

        fmr.fmr_device = info->gfi_dev;
        fmr.fmr_physical = rec_fsblk;
        fmr.fmr_owner = rec->fmr_owner;
        fmr.fmr_flags = FMR_OF_SPECIAL_OWNER;
        fmr.fmr_length = rec->fmr_length;
        error = info->gfi_formatter(&fmr, info->gfi_format_arg);
        if (error)
                return error;
        info->gfi_head->fmh_entries++;

out:
        rec_fsblk += rec->fmr_length;
        if (info->gfi_next_fsblk < rec_fsblk)
                info->gfi_next_fsblk = rec_fsblk;
        return EXT4_QUERY_RANGE_CONTINUE;
}

static inline ext4_fsblk_t ext4_fsmap_next_pblk(struct ext4_fsmap *fmr)
{
        return fmr->fmr_physical + fmr->fmr_length;
}

/* Transform a blockgroup's free record into a fsmap */
static int ext4_getfsmap_datadev_helper(struct super_block *sb,
                                        ext4_group_t agno, ext4_grpblk_t start,
                                        ext4_grpblk_t len, void *priv)
{
        struct ext4_fsmap irec;
        struct ext4_getfsmap_info *info = priv;
        struct ext4_fsmap *p;
        struct ext4_fsmap *tmp;
        struct ext4_sb_info *sbi = EXT4_SB(sb);
        ext4_fsblk_t fsb;
        ext4_fsblk_t fslen;
        int error;

        fsb = (EXT4_C2B(sbi, start) + ext4_group_first_block_no(sb, agno));
        fslen = EXT4_C2B(sbi, len);

        /* If the retained free extent record is set... */
        if (info->gfi_lastfree.fmr_owner) {
                /* ...and abuts this one, lengthen it and return. */
                if (ext4_fsmap_next_pblk(&info->gfi_lastfree) == fsb) {
                        info->gfi_lastfree.fmr_length += fslen;
                        return 0;
                }

                /*
                 * There's a gap between the two free extents; emit the
                 * retained extent prior to merging the meta_list.
                 */
                error = ext4_getfsmap_helper(sb, info, &info->gfi_lastfree);
                if (error)
                        return error;
                info->gfi_lastfree.fmr_owner = 0;
        }

        /* Merge in any relevant extents from the meta_list */
        list_for_each_entry_safe(p, tmp, &info->gfi_meta_list, fmr_list) {
                if (p->fmr_physical + p->fmr_length <= info->gfi_next_fsblk) {
                        list_del(&p->fmr_list);
                        kfree(p);
                } else if (p->fmr_physical < fsb) {
                        error = ext4_getfsmap_helper(sb, info, p);
                        if (error)
                                return error;

                        list_del(&p->fmr_list);
                        kfree(p);
                }
        }

        irec.fmr_device = 0;
        irec.fmr_physical = fsb;
        irec.fmr_length = fslen;
        irec.fmr_owner = EXT4_FMR_OWN_FREE;
        irec.fmr_flags = 0;

        /* If this is a free extent at the end of a bg, buffer it. */
        if (ext4_fsmap_next_pblk(&irec) ==
                        ext4_group_first_block_no(sb, agno + 1)) {
                info->gfi_lastfree = irec;
                return 0;
        }

        /* Otherwise, emit it */
        return ext4_getfsmap_helper(sb, info, &irec);
}

/* Execute a getfsmap query against the log device. */
static int ext4_getfsmap_logdev(struct super_block *sb, struct ext4_fsmap *keys,
                                struct ext4_getfsmap_info *info)
{
        journal_t *journal = EXT4_SB(sb)->s_journal;
        struct ext4_fsmap irec;

        /* Set up search keys */
        info->gfi_low = keys[0];
        info->gfi_low.fmr_length = 0;

        memset(&info->gfi_high, 0xFF, sizeof(info->gfi_high));

        trace_ext4_fsmap_low_key(sb, info->gfi_dev, 0,
                        info->gfi_low.fmr_physical,
                        info->gfi_low.fmr_length,
                        info->gfi_low.fmr_owner);

        trace_ext4_fsmap_high_key(sb, info->gfi_dev, 0,
                        info->gfi_high.fmr_physical,
                        info->gfi_high.fmr_length,
                        info->gfi_high.fmr_owner);

        if (keys[0].fmr_physical > 0)
                return 0;

        /* Fabricate an rmap entry for the external log device. */
        irec.fmr_physical = journal->j_blk_offset;
        irec.fmr_length = journal->j_maxlen;
        irec.fmr_owner = EXT4_FMR_OWN_LOG;
        irec.fmr_flags = 0;

        return ext4_getfsmap_helper(sb, info, &irec);
}

/* Helper to fill out an ext4_fsmap. */
static inline int ext4_getfsmap_fill(struct list_head *meta_list,
                                     ext4_fsblk_t fsb, ext4_fsblk_t len,
                                     uint64_t owner)
{
        struct ext4_fsmap *fsm;

        fsm = kmalloc(sizeof(*fsm), GFP_NOFS);
        if (!fsm)
                return -ENOMEM;
        fsm->fmr_device = 0;
        fsm->fmr_flags = 0;
        fsm->fmr_physical = fsb;
        fsm->fmr_owner = owner;
        fsm->fmr_length = len;
        list_add_tail(&fsm->fmr_list, meta_list);

        return 0;
}

/*
 * This function returns the number of file system metadata blocks at
 * the beginning of a block group, including the reserved gdt blocks.
 */
static unsigned int ext4_getfsmap_find_sb(struct super_block *sb,
                                          ext4_group_t agno,
                                          struct list_head *meta_list)
{
        struct ext4_sb_info *sbi = EXT4_SB(sb);
        ext4_fsblk_t fsb = ext4_group_first_block_no(sb, agno);
        ext4_fsblk_t len;
        unsigned long first_meta_bg = le32_to_cpu(sbi->s_es->s_first_meta_bg);
        unsigned long metagroup = agno / EXT4_DESC_PER_BLOCK(sb);
        int error;

        /* Record the superblock. */
        if (ext4_bg_has_super(sb, agno)) {
                error = ext4_getfsmap_fill(meta_list, fsb, 1, EXT4_FMR_OWN_FS);
                if (error)
                        return error;
                fsb++;
        }

        /* Record the group descriptors. */
        len = ext4_bg_num_gdb(sb, agno);
        if (!len)
                return 0;
        error = ext4_getfsmap_fill(meta_list, fsb, len,
                                   EXT4_FMR_OWN_GDT);
        if (error)
                return error;
        fsb += len;

        /* Reserved GDT blocks */
        if (!ext4_has_feature_meta_bg(sb) || metagroup < first_meta_bg) {
                len = le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks);
                error = ext4_getfsmap_fill(meta_list, fsb, len,
                                           EXT4_FMR_OWN_RESV_GDT);
                if (error)
                        return error;
        }

        return 0;
}

/* Compare two fsmap items. */
static int ext4_getfsmap_compare(void *priv,
                                 struct list_head *a,
                                 struct list_head *b)
{
        struct ext4_fsmap *fa;
        struct ext4_fsmap *fb;

        fa = container_of(a, struct ext4_fsmap, fmr_list);
        fb = container_of(b, struct ext4_fsmap, fmr_list);
        if (fa->fmr_physical < fb->fmr_physical)
                return -1;
        else if (fa->fmr_physical > fb->fmr_physical)
                return 1;
        return 0;
}

/* Merge adjacent extents of fixed metadata. */
static void ext4_getfsmap_merge_fixed_metadata(struct list_head *meta_list)
{
        struct ext4_fsmap *p;
        struct ext4_fsmap *prev = NULL;
        struct ext4_fsmap *tmp;

        list_for_each_entry_safe(p, tmp, meta_list, fmr_list) {
                if (!prev) {
                        prev = p;
                        continue;
                }

                if (prev->fmr_owner == p->fmr_owner &&
                    prev->fmr_physical + prev->fmr_length == p->fmr_physical) {
                        prev->fmr_length += p->fmr_length;
                        list_del(&p->fmr_list);
                        kfree(p);
                } else
                        prev = p;
        }
}

/* Free a list of fixed metadata. */
static void ext4_getfsmap_free_fixed_metadata(struct list_head *meta_list)
{
        struct ext4_fsmap *p;
        struct ext4_fsmap *tmp;

        list_for_each_entry_safe(p, tmp, meta_list, fmr_list) {
                list_del(&p->fmr_list);
                kfree(p);
        }
}

/* Find all the fixed metadata in the filesystem. */
int ext4_getfsmap_find_fixed_metadata(struct super_block *sb,
                                      struct list_head *meta_list)
{
        struct ext4_group_desc *gdp;
        ext4_group_t agno;
        int error;

        INIT_LIST_HEAD(meta_list);

        /* Collect everything. */
        for (agno = 0; agno < EXT4_SB(sb)->s_groups_count; agno++) {
                gdp = ext4_get_group_desc(sb, agno, NULL);
                if (!gdp) {
                        error = -EFSCORRUPTED;
                        goto err;
                }

                /* Superblock & GDT */
                error = ext4_getfsmap_find_sb(sb, agno, meta_list);
                if (error)
                        goto err;

                /* Block bitmap */
                error = ext4_getfsmap_fill(meta_list,
                                           ext4_block_bitmap(sb, gdp), 1,
                                           EXT4_FMR_OWN_BLKBM);
                if (error)
                        goto err;

                /* Inode bitmap */
                error = ext4_getfsmap_fill(meta_list,
                                           ext4_inode_bitmap(sb, gdp), 1,
                                           EXT4_FMR_OWN_INOBM);
                if (error)
                        goto err;

                /* Inodes */
                error = ext4_getfsmap_fill(meta_list,
                                           ext4_inode_table(sb, gdp),
                                           EXT4_SB(sb)->s_itb_per_group,
                                           EXT4_FMR_OWN_INODES);
                if (error)
                        goto err;
        }

        /* Sort the list */
        list_sort(NULL, meta_list, ext4_getfsmap_compare);

        /* Merge adjacent extents */
        ext4_getfsmap_merge_fixed_metadata(meta_list);

        return 0;
err:
        ext4_getfsmap_free_fixed_metadata(meta_list);
        return error;
}

/* Execute a getfsmap query against the buddy bitmaps */
static int ext4_getfsmap_datadev(struct super_block *sb,
                                 struct ext4_fsmap *keys,
                                 struct ext4_getfsmap_info *info)
{
        struct ext4_sb_info *sbi = EXT4_SB(sb);
        ext4_fsblk_t start_fsb;
        ext4_fsblk_t end_fsb;
        ext4_fsblk_t bofs;
        ext4_fsblk_t eofs;
        ext4_group_t start_ag;
        ext4_group_t end_ag;
        ext4_grpblk_t first_cluster;
        ext4_grpblk_t last_cluster;
        int error = 0;

        bofs = le32_to_cpu(sbi->s_es->s_first_data_block);
        eofs = ext4_blocks_count(sbi->s_es);
        if (keys[0].fmr_physical >= eofs)
                return 0;
        else if (keys[0].fmr_physical < bofs)
                keys[0].fmr_physical = bofs;
        if (keys[1].fmr_physical >= eofs)
                keys[1].fmr_physical = eofs - 1;
        start_fsb = keys[0].fmr_physical;
        end_fsb = keys[1].fmr_physical;

        /* Determine first and last group to examine based on start and end */
        ext4_get_group_no_and_offset(sb, start_fsb, &start_ag, &first_cluster);
        ext4_get_group_no_and_offset(sb, end_fsb, &end_ag, &last_cluster);

        /*
         * Convert the fsmap low/high keys to bg based keys.  Initialize
         * low to the fsmap low key and max out the high key to the end
         * of the bg.
         */
        info->gfi_low = keys[0];
        info->gfi_low.fmr_physical = EXT4_C2B(sbi, first_cluster);
        info->gfi_low.fmr_length = 0;

        memset(&info->gfi_high, 0xFF, sizeof(info->gfi_high));

        /* Assemble a list of all the fixed-location metadata. */
        error = ext4_getfsmap_find_fixed_metadata(sb, &info->gfi_meta_list);
        if (error)
                goto err;

        /* Query each bg */
        for (info->gfi_agno = start_ag;
             info->gfi_agno <= end_ag;
             info->gfi_agno++) {
                /*
                 * Set the bg high key from the fsmap high key if this
                 * is the last bg that we're querying.
                 */
                if (info->gfi_agno == end_ag) {
                        info->gfi_high = keys[1];
                        info->gfi_high.fmr_physical = EXT4_C2B(sbi,
                                        last_cluster);
                        info->gfi_high.fmr_length = 0;
                }

                trace_ext4_fsmap_low_key(sb, info->gfi_dev, info->gfi_agno,
                                info->gfi_low.fmr_physical,
                                info->gfi_low.fmr_length,
                                info->gfi_low.fmr_owner);

                trace_ext4_fsmap_high_key(sb, info->gfi_dev, info->gfi_agno,
                                info->gfi_high.fmr_physical,
                                info->gfi_high.fmr_length,
                                info->gfi_high.fmr_owner);

                error = ext4_mballoc_query_range(sb, info->gfi_agno,
                                EXT4_B2C(sbi, info->gfi_low.fmr_physical),
                                EXT4_B2C(sbi, info->gfi_high.fmr_physical),
                                ext4_getfsmap_datadev_helper, info);
                if (error)
                        goto err;

                /*
                 * Set the bg low key to the start of the bg prior to
                 * moving on to the next bg.
                 */
                if (info->gfi_agno == start_ag)
                        memset(&info->gfi_low, 0, sizeof(info->gfi_low));
        }

        /* Do we have a retained free extent? */
        if (info->gfi_lastfree.fmr_owner) {
                error = ext4_getfsmap_helper(sb, info, &info->gfi_lastfree);
                if (error)
                        goto err;
        }

        /* Report any gaps at the end of the bg */
        info->gfi_last = true;
        error = ext4_getfsmap_datadev_helper(sb, end_ag, last_cluster, 0, info);
        if (error)
                goto err;

err:
        ext4_getfsmap_free_fixed_metadata(&info->gfi_meta_list);
        return error;
}

/* Do we recognize the device? */
static bool ext4_getfsmap_is_valid_device(struct super_block *sb,
                                          struct ext4_fsmap *fm)
{
        if (fm->fmr_device == 0 || fm->fmr_device == UINT_MAX ||
            fm->fmr_device == new_encode_dev(sb->s_bdev->bd_dev))
                return true;
        if (EXT4_SB(sb)->journal_bdev &&
            fm->fmr_device == new_encode_dev(EXT4_SB(sb)->journal_bdev->bd_dev))
                return true;
        return false;
}

/* Ensure that the low key is less than the high key. */
static bool ext4_getfsmap_check_keys(struct ext4_fsmap *low_key,
                                     struct ext4_fsmap *high_key)
{
        if (low_key->fmr_device > high_key->fmr_device)
                return false;
        if (low_key->fmr_device < high_key->fmr_device)
                return true;

        if (low_key->fmr_physical > high_key->fmr_physical)
                return false;
        if (low_key->fmr_physical < high_key->fmr_physical)
                return true;

        if (low_key->fmr_owner > high_key->fmr_owner)
                return false;
        if (low_key->fmr_owner < high_key->fmr_owner)
                return true;

        return false;
}

#define EXT4_GETFSMAP_DEVS      2
/*
 * Get filesystem's extents as described in head, and format for
 * output.  Calls formatter to fill the user's buffer until all
 * extents are mapped, until the passed-in head->fmh_count slots have
 * been filled, or until the formatter short-circuits the loop, if it
 * is tracking filled-in extents on its own.
 *
 * Key to Confusion
 * ----------------
 * There are multiple levels of keys and counters at work here:
 * _fsmap_head.fmh_keys         -- low and high fsmap keys passed in;
 *                                 these reflect fs-wide block addrs.
 * dkeys                        -- fmh_keys used to query each device;
 *                                 these are fmh_keys but w/ the low key
 *                                 bumped up by fmr_length.
 * _getfsmap_info.gfi_next_fsblk-- next fs block we expect to see; this
 *                                 is how we detect gaps in the fsmap
 *                                 records and report them.
 * _getfsmap_info.gfi_low/high  -- per-bg low/high keys computed from
 *                                 dkeys; used to query the free space.
 */
int ext4_getfsmap(struct super_block *sb, struct ext4_fsmap_head *head,
                  ext4_fsmap_format_t formatter, void *arg)
{
        struct ext4_fsmap dkeys[2];     /* per-dev keys */
        struct ext4_getfsmap_dev handlers[EXT4_GETFSMAP_DEVS];
        struct ext4_getfsmap_info info = {0};
        int i;
        int error = 0;

        if (head->fmh_iflags & ~FMH_IF_VALID)
                return -EINVAL;
        if (!ext4_getfsmap_is_valid_device(sb, &head->fmh_keys[0]) ||
            !ext4_getfsmap_is_valid_device(sb, &head->fmh_keys[1]))
                return -EINVAL;

        head->fmh_entries = 0;

        /* Set up our device handlers. */
        memset(handlers, 0, sizeof(handlers));
        handlers[0].gfd_dev = new_encode_dev(sb->s_bdev->bd_dev);
        handlers[0].gfd_fn = ext4_getfsmap_datadev;
        if (EXT4_SB(sb)->journal_bdev) {
                handlers[1].gfd_dev = new_encode_dev(
                                EXT4_SB(sb)->journal_bdev->bd_dev);
                handlers[1].gfd_fn = ext4_getfsmap_logdev;
        }

        sort(handlers, EXT4_GETFSMAP_DEVS, sizeof(struct ext4_getfsmap_dev),
                        ext4_getfsmap_dev_compare, NULL);

        /*
         * To continue where we left off, we allow userspace to use the
         * last mapping from a previous call as the low key of the next.
         * This is identified by a non-zero length in the low key. We
         * have to increment the low key in this scenario to ensure we
         * don't return the same mapping again, and instead return the
         * very next mapping.
         *
         * Bump the physical offset as there can be no other mapping for
         * the same physical block range.
         */
        dkeys[0] = head->fmh_keys[0];
        dkeys[0].fmr_physical += dkeys[0].fmr_length;
        dkeys[0].fmr_owner = 0;
        dkeys[0].fmr_length = 0;
        memset(&dkeys[1], 0xFF, sizeof(struct ext4_fsmap));

        if (!ext4_getfsmap_check_keys(dkeys, &head->fmh_keys[1]))
                return -EINVAL;

        info.gfi_next_fsblk = head->fmh_keys[0].fmr_physical +
                          head->fmh_keys[0].fmr_length;
        info.gfi_formatter = formatter;
        info.gfi_format_arg = arg;
        info.gfi_head = head;

        /* For each device we support... */
        for (i = 0; i < EXT4_GETFSMAP_DEVS; i++) {
                /* Is this device within the range the user asked for? */
                if (!handlers[i].gfd_fn)
                        continue;
                if (head->fmh_keys[0].fmr_device > handlers[i].gfd_dev)
                        continue;
                if (head->fmh_keys[1].fmr_device < handlers[i].gfd_dev)
                        break;

                /*
                 * If this device number matches the high key, we have
                 * to pass the high key to the handler to limit the
                 * query results.  If the device number exceeds the
                 * low key, zero out the low key so that we get
                 * everything from the beginning.
                 */
                if (handlers[i].gfd_dev == head->fmh_keys[1].fmr_device)
                        dkeys[1] = head->fmh_keys[1];
                if (handlers[i].gfd_dev > head->fmh_keys[0].fmr_device)
                        memset(&dkeys[0], 0, sizeof(struct ext4_fsmap));

                info.gfi_dev = handlers[i].gfd_dev;
                info.gfi_last = false;
                info.gfi_agno = -1;
                error = handlers[i].gfd_fn(sb, dkeys, &info);
                if (error)
                        break;
                info.gfi_next_fsblk = 0;
        }

        head->fmh_oflags = FMH_OF_DEV_T;
        return error;
}