Re: [PATCH] ext4: fast commit: avoid fs_reclaim inversion in perform_commit

[Jan Kara]

On Tue 23-12-25 21:13:42, Li Chen wrote:
> lockdep reports a possible deadlock due to lock order inversion:
> 
>      CPU0                    CPU1
>      ----                    ----
> lock(fs_reclaim);
>                              lock(&sbi->s_fc_lock);
>                              lock(fs_reclaim);
> lock(&sbi->s_fc_lock);
> 
> ext4_fc_perform_commit() holds s_fc_lock while writing the fast commit
> log. Allocations here can enter reclaim and take fs_reclaim, inverting
> with ext4_fc_del() which runs under fs_reclaim during inode eviction.
> Wrap Step 6 in memalloc_nofs_save()/restore() so reclaim is skipped
> while s_fc_lock is held.
> 
> Fixes: 6593714d67ba ("ext4: hold s_fc_lock while during fast commit")
> Signed-off-by: Li Chen <me@linux.beauty>

Thanks for the analysis and the patch! Your solution is in principle
correct but it's a bit fragile because there can be other instances (or we
can grow them in the future) where sbi->s_fc_lock is held when doing
allocation. The situation is that sbi->s_fc_lock can be acquired from inode
eviction path (ext4_clear_inode()) and thus this lock is inherently reclaim
unsafe. What we do in such cases is that we create helper functions for
acquiring / releasing the lock while also setting proper context and using
these helpers - like in commit 00d873c17e29 ("ext4: avoid deadlock in fs
reclaim with page writeback"). Can you perhaps modify your patch to follow
that behavior as well?

								Honza

> diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
> index 3bcdd4619de1..b0c458082997 100644
> --- a/fs/ext4/fast_commit.c
> +++ b/fs/ext4/fast_commit.c
> @@ -1045,6 +1045,7 @@ static int ext4_fc_perform_commit(journal_t *journal)
>  	struct ext4_fc_head head;
>  	struct inode *inode;
>  	struct blk_plug plug;
> +	unsigned int nofs;
>  	int ret = 0;
>  	u32 crc = 0;
>  
> @@ -1118,6 +1119,7 @@ static int ext4_fc_perform_commit(journal_t *journal)
>  		blkdev_issue_flush(journal->j_fs_dev);
>  
>  	blk_start_plug(&plug);
> +	nofs = memalloc_nofs_save();
>  	/* Step 6: Write fast commit blocks to disk. */
>  	if (sbi->s_fc_bytes == 0) {
>  		/*
> @@ -1158,6 +1160,7 @@ static int ext4_fc_perform_commit(journal_t *journal)
>  
>  out:
>  	mutex_unlock(&sbi->s_fc_lock);
> +	memalloc_nofs_restore(nofs);
>  	blk_finish_plug(&plug);
>  	return ret;
>  }
> -- 
> 2.52.0
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR