[f2fs-dev] [PATCH v3 1/2] f2fs: fix zero-sized extent for precache extents

[wangzijie]

Script to reproduce:
f2fs_io write 1 0 1881 rand dsync testfile
f2fs_io fallocate 0 7708672 4096 testfile
f2fs_io write 1 1881 1 rand buffered testfile
fsync testfile
umount
mount
f2fs_io precache_extents testfile

When the data layout is something like this:
dnode1:                     dnode2:
[0]      A                  [0]    NEW_ADDR
[1]      A+1                [1]    0x0
...
[1016]   A+1016
[1017]   B (B!=A+1017)      [1017] 0x0

During precache_extents, we map the last block(valid blkaddr) in dnode1:
map->m_flags |= F2FS_MAP_MAPPED;
map->m_pblk = blkaddr(valid blkaddr);
map->m_len = 1;
then we goto next_dnode, meet the first block in dnode2(hole), goto sync_out:
map->m_flags & F2FS_MAP_MAPPED == true, and we make zero-sized extent:

map->m_len = 1
ofs = start_pgofs - map->m_lblk = 1882 - 1881 = 1
ei.fofs = start_pgofs = 1882
ei.len = map->m_len - ofs = 1 - 1 = 0


Rebased on patch[1], this patch can cover these cases to avoid zero-sized extent:
A,B,C is valid blkaddr
case1:
dnode1:                     dnode2:
[0]      A                  [0]    NEW_ADDR
[1]      A+1                [1]    0x0
...                         ....
[1016]   A+1016
[1017]   B (B!=A+1017)      [1017] 0x0

case2:
dnode1:                     dnode2:
[0]      A                  [0]    C (C!=B+1)
[1]      A+1                [1]    C+1
...                         ....
[1016]   A+1016
[1017]   B (B!=A+1017)      [1017] 0x0

case3:
dnode1:                     dnode2:
[0]      A                  [0]    C (C!=B+2)
[1]      A+1                [1]    C+1
...                         ....
[1015]   A+1015
[1016]   B (B!=A+1016)
[1017]   B+1                [1017] 0x0

[1] https://lore.kernel.org/linux-f2fs-devel/20250912081250.44383-1-chao@xxxxxxxxxx/

Fixes: c4020b2da4c9 ("f2fs: support F2FS_IOC_PRECACHE_EXTENTS")
Signed-off-by: wangzijie <wangzijie1@xxxxxxxxx>
---
Rebased on:
https://lore.kernel.org/linux-f2fs-devel/20250912081250.44383-1-chao@xxxxxxxxxx/
v2:
https://lore.kernel.org/linux-f2fs-devel/20250915035246.98055-1-wangzijie1@xxxxxxxxx/
---
 fs/f2fs/data.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 838eae39d..7a5170b32 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -1778,9 +1778,10 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map, int flag)
 		if (map->m_flags & F2FS_MAP_MAPPED) {
 			unsigned int ofs = start_pgofs - map->m_lblk;
 
-			f2fs_update_read_extent_cache_range(&dn,
-				start_pgofs, map->m_pblk + ofs,
-				map->m_len - ofs);
+			if (map->m_len - ofs > 0)
+				f2fs_update_read_extent_cache_range(&dn,
+					start_pgofs, map->m_pblk + ofs,
+					map->m_len - ofs);
 		}
 		if (map->m_next_extent)
 			*map->m_next_extent = is_hole ? pgofs + 1 : pgofs;
-- 
2.25.1