Re: [PATCH] ext4: Fix calculation of credits for extent tree modification

[Zhang Yi <yi.zhang@huawei.com>]

Hi, Jan!

On 2025/4/30 1:55, Jan Kara wrote:
> Luis and David are reporting that after running generic/750 test for 90+
> hours on 2k ext4 filesystem, they are able to trigger a warning in
> jbd2_journal_dirty_metadata() complaining that there are not enough
> credits in the running transaction started in ext4_do_writepages().
> 
> Indeed the code in ext4_do_writepages() is racy and the extent tree can
> change between the time we compute credits necessary for extent tree
> computation and the time we actually modify the extent tree. Thus it may
> happen that the number of credits actually needed is higher. Modify
> ext4_ext_index_trans_blocks() to count with the worst case of maximum
> tree depth. This can reduce the possible number of writers that can
> operate in the system in parallel (because the credit estimates now won't
> fit in one transaction) but for reasonably sized journals this shouldn't
> really be an issue. So just go with a safe and simple fix.
> 
> Link: https://lore.kernel.org/all/20250415013641.f2ppw6wov4kn4wq2@offworld
> Reported-by: Davidlohr Bueso <dave@stgolabs.net>
> Reported-by: Luis Chamberlain <mcgrof@kernel.org>
> Tested-by: kdevops@lists.linux.dev
> Signed-off-by: Jan Kara <jack@suse.cz>

This overall looks good to me now. However, the credit calculation in
ext4_ext_index_trans_blocks() seems still appears to be incorrect
because it does not include the leaf extent blocks. I discovered this
problem while attempting to enable large folios for ext4. It can easily
trigger problems when writing back a 2MB folio with a 1K block size,
and each block is discontinuous.

  https://lore.kernel.org/linux-ext4/20241125114419.903270-7-yi.zhang@huaweicloud.com/

Fortunately, this problem can only triggered after we support large
folio.

Reviewed-by: Zhang Yi <yi.zhang@huawei.com>

> ---
>  fs/ext4/extents.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index c616a16a9f36..43286632e650 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -2396,18 +2396,19 @@ int ext4_ext_calc_credits_for_single_extent(struct inode *inode, int nrblocks,
>  int ext4_ext_index_trans_blocks(struct inode *inode, int extents)
>  {
>  	int index;
> -	int depth;
>  
>  	/* If we are converting the inline data, only one is needed here. */
>  	if (ext4_has_inline_data(inode))
>  		return 1;
>  
> -	depth = ext_depth(inode);
> -
> +	/*
> +	 * Extent tree can change between the time we estimate credits and
> +	 * the time we actually modify the tree. Assume the worst case.
> +	 */
>  	if (extents <= 1)
> -		index = depth * 2;
> +		index = EXT4_MAX_EXTENT_DEPTH * 2;
>  	else
> -		index = depth * 3;
> +		index = EXT4_MAX_EXTENT_DEPTH * 3;
>  
>  	return index;
>  }