[PATCH v2] f2fs: fix i_size when pinned fallocate partially fails

[Zhan Xusheng]

From: Zhan Xusheng <zhanxusheng1024@xxxxxxxxx>

From: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>

Commit 4275b59673eb ("f2fs: fix to round down start offset of fallocate
for pin file") moved the allocation loop's start down to a section
boundary, but the error path still converts @expanded against @pg_start,
which holds the unrounded start.

@pg_start exists for that conversion: commit 88f2cfc5fa90 ("f2fs: fix to
update last i_size if fallocate partially succeeds") added it as an
immutable base because map.m_lblk moves every round.  Each round now maps
exactly sec_blks blocks starting from rounddown(pg_start, sec_blks), so
pg_start + expanded overshoots the last allocated block by
pg_start % sec_blks, and a partial failure leaves i_size covering a tail
that was never allocated.  Nothing corrects that afterwards either, since
file_dont_truncate() has already cleared FADVISE_TRUNC_BIT.

It needs a start offset that is not section aligned plus a fallocate that
hits ENOSPC partway, so the error path runs with expanded > 0.  On an
80 MiB image with 2 MiB sections:

  truncate -s 80M img
  mkfs.f2fs -s 1 -f img
  mount -o loop img /mnt
  touch /mnt/pinned
  f2fs_io pinfile set /mnt/pinned
  # 2093056 = block 511, so pg_start % sec_blks = 511
  f2fs_io fallocate 0 2093056 536870912 /mnt/pinned
  stat -c %s /mnt/pinned
  filefrag -v /mnt/pinned

The last extent ends at block 10737 either way.  Before, i_size is
46075904, block 11249, so 511 blocks of it were never allocated, and
filefrag does not mark the last extent eof.  After, i_size is 43982848,
block 10738, and eof is back.  A kernel from before that commit also
shows no overshoot.

Keep @pg_start pointing at where allocation actually begins.

Fixes: 4275b59673eb ("f2fs: fix to round down start offset of fallocate for pin file")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
---
v1->v2:
- Added the reproducer and the before/after numbers to the changelog,
  as asked by Chao Yu.  No code change.

v1: https://lore.kernel.org/r/20260817022723.3324123-1-zhanxusheng@xxxxxxxxxx

 fs/f2fs/file.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 4b52c56d71f0..cc0d2b8c4684 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -1919,8 +1919,9 @@ static int f2fs_expand_inode_data(struct inode *inode, loff_t offset,
 		block_t sec_len;
 
 		if (map.m_lblk % sec_blks) {
-			map.m_lblk = rounddown(map.m_lblk, sec_blks);
-			map.m_len = pg_end - map.m_lblk;
+			pg_start = rounddown(map.m_lblk, sec_blks);
+			map.m_lblk = pg_start;
+			map.m_len = pg_end - pg_start;
 			if (off_end)
 				map.m_len++;
 		}
-- 
2.43.0