[PATCH] f2fs: fix to avoid mapping wrong physical block for swapfile

[PATCH] f2fs: fix to avoid mapping wrong physical block for swapfile

[Chao Yu]

Xiaolong Guo reported a f2fs bug in bugzilla [1]

[1] https://bugzilla.kernel.org/show_bug.cgi?id=220951

Quoted:

"When using stress-ng's swap stress test on F2FS filesystem with kernel 6.6+,
the system experiences data corruption leading to either:
1 dm-verity corruption errors and device reboot
2 F2FS node corruption errors and boot hangs

The issue occurs specifically when:
1 Using F2FS filesystem (ext4 is unaffected)
2 Swapfile size is less than F2FS section size (2MB)
3 Swapfile has fragmented physical layout (multiple non-contiguous extents)
4 Kernel version is 6.6+ (6.1 is unaffected)

The root cause is in check_swap_activate() function in fs/f2fs/data.c. When the
first extent of a small swapfile (< 2MB) is not aligned to section boundaries,
the function incorrectly treats it as the last extent, failing to map
subsequent extents. This results in incorrect swap_extent creation where only
the first extent is mapped, causing subsequent swap writes to overwrite wrong
physical locations (other files' data).

Steps to Reproduce
1 Setup a device with F2FS-formatted userdata partition
2 Compile stress-ng from https://github.com/ColinIanKing/stress-ng
3 Run swap stress test: (Android devices)
adb shell "cd /data/stressng; ./stress-ng-64 --metrics-brief --timeout 60
--swap 0"

Log:
1 Ftrace shows in kernel 6.6, only first extent is mapped during second
f2fs_map_blocks call in check_swap_activate():
stress-ng-swap-8990: f2fs_map_blocks: ino=11002, file offset=0, start
blkaddr=0x43143, len=0x1
(Only 4KB mapped, not the full swapfile)
2 in kernel 6.1, both extents are correctly mapped:
stress-ng-swap-5966: f2fs_map_blocks: ino=28011, file offset=0, start
blkaddr=0x13cd4, len=0x1
stress-ng-swap-5966: f2fs_map_blocks: ino=28011, file offset=1, start
blkaddr=0x60c84b, len=0xff

The problematic code is in check_swap_activate():
if ((pblock - SM_I(sbi)->main_blkaddr) % blks_per_sec ||
    nr_pblocks % blks_per_sec ||
    !f2fs_valid_pinned_area(sbi, pblock)) {
    bool last_extent = false;

    not_aligned++;

    nr_pblocks = roundup(nr_pblocks, blks_per_sec);
    if (cur_lblock + nr_pblocks > sis->max)
        nr_pblocks -= blks_per_sec;

    /* this extent is last one */
    if (!nr_pblocks) {
        nr_pblocks = last_lblock - cur_lblock;
        last_extent = true;
    }

    ret = f2fs_migrate_blocks(inode, cur_lblock, nr_pblocks);
    if (ret) {
        if (ret == -ENOENT)
            ret = -EINVAL;
        goto out;
    }

    if (!last_extent)
        goto retry;
}

When the first extent is unaligned and roundup(nr_pblocks, blks_per_sec)
exceeds sis->max, we subtract blks_per_sec resulting in nr_pblocks = 0. The
code then incorrectly assumes this is the last extent, sets nr_pblocks =
last_lblock - cur_lblock (entire swapfile), and performs migration. After
migration, it doesn't retry mapping, so subsequent extents are never processed.
"

In order to fix this issue, we need to lookup block mapping info after
we migrate all blocks in the tail of swapfile.

Cc: stable@kernel.org
Fixes: 9703d69d9d15 ("f2fs: support file pinning for zoned devices")
Cc: Daeho Jeong <daehojeong@google.com>
Reported-and-tested-by: Xiaolong Guo <guoxiaolong2008@gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220951
Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/data.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index eea6311998c1..ed5e28352a92 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -4196,6 +4196,7 @@ static int check_swap_activate(struct swap_info_struct *sis,
 
 	while (cur_lblock < last_lblock && cur_lblock < sis->max) {
 		struct f2fs_map_blocks map;
+		bool last_extent = false;
 retry:
 		cond_resched();
 
@@ -4221,11 +4222,10 @@ static int check_swap_activate(struct swap_info_struct *sis,
 		pblock = map.m_pblk;
 		nr_pblocks = map.m_len;
 
-		if ((pblock - SM_I(sbi)->main_blkaddr) % blks_per_sec ||
-				nr_pblocks % blks_per_sec ||
-				f2fs_is_sequential_zone_area(sbi, pblock)) {
-			bool last_extent = false;
-
+		if (!last_extent &&
+			((pblock - SM_I(sbi)->main_blkaddr) % blks_per_sec ||
+			nr_pblocks % blks_per_sec ||
+			f2fs_is_sequential_zone_area(sbi, pblock))) {
 			not_aligned++;
 
 			nr_pblocks = roundup(nr_pblocks, blks_per_sec);
@@ -4246,8 +4246,8 @@ static int check_swap_activate(struct swap_info_struct *sis,
 				goto out;
 			}
 
-			if (!last_extent)
-				goto retry;
+			/* lookup block mapping info after block migration */
+			goto retry;
 		}
 
 		if (cur_lblock + nr_pblocks >= sis->max)
-- 
2.40.1

Re: [f2fs-dev] [PATCH] f2fs: fix to avoid mapping wrong physical block for swapfile

[patchwork-bot+f2fs]

Hello:

This patch was applied to jaegeuk/f2fs.git (dev)
by Jaegeuk Kim <jaegeuk@kernel.org>:

On Tue, 13 Jan 2026 14:22:29 +0800 you wrote:
> Xiaolong Guo reported a f2fs bug in bugzilla [1]
> 
> [1] https://bugzilla.kernel.org/show_bug.cgi?id=220951
> 
> Quoted:
> 
> "When using stress-ng's swap stress test on F2FS filesystem with kernel 6.6+,
> the system experiences data corruption leading to either:
> 1 dm-verity corruption errors and device reboot
> 2 F2FS node corruption errors and boot hangs
> 
> [...]

Here is the summary with links:
  - [f2fs-dev] f2fs: fix to avoid mapping wrong physical block for swapfile
    https://git.kernel.org/jaegeuk/f2fs/c/5c145c03188b

You are awesome, thank you!
-- 
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