[f2fs-dev] [PATCH v2] f2fs: quota: fix quota flush failure during filesystem freeze

[f2fs-dev] [PATCH v2] f2fs: quota: fix quota flush failure during filesystem freeze

[Jianan Huang]

Quota flush retries can be exhausted during filesystem freeze.

freeze_super() and the quota checkpoint may race as below.

freeze_super()                       f2fs_ckpt
- down_write(&sb->s_umount)
- sb->s_writers.frozen = SB_FREEZE_PAGEFAULT
- sync_filesystem()
 - f2fs_sync_fs()
  - f2fs_issue_checkpoint()
   - queue CP_SYNC -----------------> - block_operations()
                                      - down_read_trylock(s_umount)
                                        : fails; freeze holds write lock
                                      - quota flush retries exhausted
                                      - set CP_QUOTA_NEED_FSCK_FLAG

During the PAGEFAULT sync pass, freeze_super() holds s_umount for
write, but umount_lock_holder is not set until f2fs_freeze().  Thus
quota writeback tries to acquire s_umount again and can exhaust retries.

With checkpoint merge enabled, f2fs_issue_checkpoint() may dispatch
CP_SYNC to f2fs_ckpt. That thread also cannot acquire the freeze lock.

Record current as umount_lock_holder around the VFS PAGEFAULT sync so
checkpoint runs in the freeze caller and quota flush uses its existing
s_umount write lock.

Fixes: eb85c2410d6f ("f2fs: quota: fix to avoid warning in dquot_writeback_dquots()")
Signed-off-by: Jianan Huang <jnhuang95@gmail.com>
---
Changes since v1:
- Add a separate VFS sync_fs wrapper to avoid renaming f2fs_sync_fs().

 fs/f2fs/super.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 00da08504d55..fa105ca14653 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -2146,6 +2146,25 @@ int f2fs_sync_fs(struct super_block *sb, int sync)
 	return err;
 }
 
+static int f2fs_sync_fs_super(struct super_block *sb, int sync)
+{
+	struct f2fs_sb_info *sbi = F2FS_SB(sb);
+	bool freeze_sync = sync &&
+			sb->s_writers.frozen == SB_FREEZE_PAGEFAULT;
+	int err;
+
+	/* freeze_super() holds s_umount for write during this sync pass. */
+	if (freeze_sync)
+		sbi->umount_lock_holder = current;
+
+	err = f2fs_sync_fs(sb, sync);
+
+	if (freeze_sync)
+		sbi->umount_lock_holder = NULL;
+
+	return err;
+}
+
 static int f2fs_freeze(struct super_block *sb)
 {
 	struct f2fs_sb_info *sbi = F2FS_SB(sb);
@@ -3757,7 +3776,7 @@ static const struct super_operations f2fs_sops = {
 #endif
 	.evict_inode	= f2fs_evict_inode,
 	.put_super	= f2fs_put_super,
-	.sync_fs	= f2fs_sync_fs,
+	.sync_fs	= f2fs_sync_fs_super,
 	.freeze_fs	= f2fs_freeze,
 	.unfreeze_fs	= f2fs_unfreeze,
 	.statfs		= f2fs_statfs,
-- 
2.43.0



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Re: [f2fs-dev] [PATCH v2] f2fs: quota: fix quota flush failure during filesystem freeze

[Chao Yu via Linux-f2fs-devel]

On 10/12/26 10:54, Jianan Huang wrote:
> Quota flush retries can be exhausted during filesystem freeze.
> 
> freeze_super() and the quota checkpoint may race as below.
> 
> freeze_super()                       f2fs_ckpt
> - down_write(&sb->s_umount)
> - sb->s_writers.frozen = SB_FREEZE_PAGEFAULT
> - sync_filesystem()
>  - f2fs_sync_fs()
>   - f2fs_issue_checkpoint()
>    - queue CP_SYNC -----------------> - block_operations()
>                                       - down_read_trylock(s_umount)
>                                         : fails; freeze holds write lock
>                                       - quota flush retries exhausted
>                                       - set CP_QUOTA_NEED_FSCK_FLAG
> 
> During the PAGEFAULT sync pass, freeze_super() holds s_umount for
> write, but umount_lock_holder is not set until f2fs_freeze().  Thus
> quota writeback tries to acquire s_umount again and can exhaust retries.
> 
> With checkpoint merge enabled, f2fs_issue_checkpoint() may dispatch
> CP_SYNC to f2fs_ckpt. That thread also cannot acquire the freeze lock.
> 
> Record current as umount_lock_holder around the VFS PAGEFAULT sync so
> checkpoint runs in the freeze caller and quota flush uses its existing
> s_umount write lock.
> 
> Fixes: eb85c2410d6f ("f2fs: quota: fix to avoid warning in dquot_writeback_dquots()")
> Signed-off-by: Jianan Huang <jnhuang95@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>

Thanks,


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Re: [f2fs-dev] [PATCH v2] f2fs: quota: fix quota flush failure during filesystem freeze

[patchwork-bot+f2fs--- via Linux-f2fs-devel]

Hello:

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

On Mon, 12 Oct 2026 10:54:42 +0800 you wrote:
> Quota flush retries can be exhausted during filesystem freeze.
> 
> freeze_super() and the quota checkpoint may race as below.
> 
> freeze_super()                       f2fs_ckpt
> - down_write(&sb->s_umount)
> - sb->s_writers.frozen = SB_FREEZE_PAGEFAULT
> - sync_filesystem()
>  - f2fs_sync_fs()
>   - f2fs_issue_checkpoint()
>    - queue CP_SYNC -----------------> - block_operations()
>                                       - down_read_trylock(s_umount)
>                                         : fails; freeze holds write lock
>                                       - quota flush retries exhausted
>                                       - set CP_QUOTA_NEED_FSCK_FLAG
> 
> [...]

Here is the summary with links:
  - [f2fs-dev,v2] f2fs: quota: fix quota flush failure during filesystem freeze
    https://git.kernel.org/jaegeuk/f2fs/c/e455328a66a5

You are awesome, thank you!
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