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ext4: correctly handle queries for metadata mappings

Currently, our handling of metadata is _ambiguous_ in some scenarios,
that is, we end up returning unknown if the range only covers the
mapping partially.

For example, in the following case:

$ xfs_io -c fsmap -d

0: 254:16 [0..7]: static fs metadata 8
1: 254:16 [8..15]: special 102:1 8
2: 254:16 [16..5127]: special 102:2 5112
3: 254:16 [5128..5255]: special 102:3 128
4: 254:16 [5256..5383]: special 102:4 128
5: 254:16 [5384..70919]: inodes 65536
6: 254:16 [70920..70967]: unknown 48
...

$ xfs_io -c fsmap -d 24 33

0: 254:16 [24..39]: unknown 16 <--- incomplete reporting

$ xfs_io -c fsmap -d 24 33 (With patch)

0: 254:16 [16..5127]: special 102:2 5112

This is because earlier in ext4_getfsmap_meta_helper, we end up ignoring
any extent that starts before our queried range, but overlaps it. While
the man page [1] is a bit ambiguous on this, this fix makes the output
make more sense since we are anyways returning an "unknown" extent. This
is also consistent to how XFS does it:

$ xfs_io -c fsmap -d

...
6: 254:16 [104..127]: free space 24
7: 254:16 [128..191]: inodes 64
...

$ xfs_io -c fsmap -d 137 150

0: 254:16 [128..191]: inodes 64 <-- full extent returned

[1] https://man7.org/linux/man-pages/man2/ioctl_getfsmap.2.html

Reported-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Message-ID: <023f37e35ee280cd9baac0296cbadcbe10995cab.1757058211.git.ojaswin@linux.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

authored by

Ojaswin Mujoo and committed by
Theodore Ts'o 46c22a8b 46e75c56

+9 -5
+9 -5
fs/ext4/fsmap.c
··· 74 74 static bool ext4_getfsmap_rec_before_low_key(struct ext4_getfsmap_info *info, 75 75 struct ext4_fsmap *rec) 76 76 { 77 - return rec->fmr_physical < info->gfi_low.fmr_physical; 77 + return rec->fmr_physical + rec->fmr_length <= 78 + info->gfi_low.fmr_physical; 78 79 } 79 80 80 81 /* ··· 201 200 ext4_group_first_block_no(sb, agno)); 202 201 fs_end = fs_start + EXT4_C2B(sbi, len); 203 202 204 - /* Return relevant extents from the meta_list */ 203 + /* 204 + * Return relevant extents from the meta_list. We emit all extents that 205 + * partially/fully overlap with the query range 206 + */ 205 207 list_for_each_entry_safe(p, tmp, &info->gfi_meta_list, fmr_list) { 206 - if (p->fmr_physical < info->gfi_next_fsblk) { 208 + if (p->fmr_physical + p->fmr_length <= info->gfi_next_fsblk) { 207 209 list_del(&p->fmr_list); 208 210 kfree(p); 209 211 continue; 210 212 } 211 - if (p->fmr_physical <= fs_start || 212 - p->fmr_physical + p->fmr_length <= fs_end) { 213 + if (p->fmr_physical <= fs_end && 214 + p->fmr_physical + p->fmr_length > fs_start) { 213 215 /* Emit the retained free extent record if present */ 214 216 if (info->gfi_lastfree.fmr_owner) { 215 217 error = ext4_getfsmap_helper(sb, info,