linux-kernel - Re: [PATCH v3 06/14] ext4: drop extent cache after doing PARTIAL

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Message-ID: <aSsud_c9UtP3Xcmm@li-dc0c254c-257c-11b2-a85c-98b6c1322444.ibm.com>
Date: Sat, 29 Nov 2025 23:03:43 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
        jack@...e.cz, yi.zhang@...wei.com, yizhang089@...il.com,
        libaokun1@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH v3 06/14] ext4: drop extent cache after doing
 PARTIAL_VALID1 zeroout

On Sat, Nov 29, 2025 at 06:32:38PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@...wei.com>
> 
> When splitting an unwritten extent in the middle and converting it to
> initialized in ext4_split_extent() with the EXT4_EXT_MAY_ZEROOUT and
> EXT4_EXT_DATA_VALID2 flags set, it could leave a stale unwritten extent.
> 
> Assume we have an unwritten file and buffered write in the middle of it
> without dioread_nolock enabled, it will allocate blocks as written
> extent.
> 
>        0  A      B  N
>        [UUUUUUUUUUUU] on-disk extent      U: unwritten extent
>        [UUUUUUUUUUUU] extent status tree
>        [--DDDDDDDD--]                     D: valid data
>           |<-  ->| ----> this range needs to be initialized
> 
> ext4_split_extent() first try to split this extent at B with
> EXT4_EXT_DATA_PARTIAL_VALID1 and EXT4_EXT_MAY_ZEROOUT flag set, but
> ext4_split_extent_at() failed to split this extent due to temporary lack
> of space. It zeroout B to N and leave the entire extent as unwritten.
> 
>        0  A      B  N
>        [UUUUUUUUUUUU] on-disk extent
>        [UUUUUUUUUUUU] extent status tree
>        [--DDDDDDDDZZ]                     Z: zeroed data
> 
> ext4_split_extent() then try to split this extent at A with
> EXT4_EXT_DATA_VALID2 flag set. This time, it split successfully and
> leave an written extent from A to N.
> 
>        0  A      B  N
>        [UUWWWWWWWWWW] on-disk extent      W: written extent
>        [UUUUUUUUUUUU] extent status tree
>        [--DDDDDDDDZZ]
> 
> Finally ext4_map_create_blocks() only insert extent A to B to the extent
> status tree, and leave an stale unwritten extent in the status tree.
> 
>        0  A      B  N
>        [UUWWWWWWWWWW] on-disk extent      W: written extent
>        [UUWWWWWWWWUU] extent status tree
>        [--DDDDDDDDZZ]
> 
> Fix this issue by always cached extent status entry after zeroing out
> the second part.
> 
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> Reviewed-by: Baokun Li <libaokun1@...wei.com>
> Cc: stable@...nel.org

Okay so now we only drop the part that would have become stale. Looks
good to me.

Reviewed-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>

Regards,
ojaswin

> ---
>  fs/ext4/extents.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index be9fd2ab8667..1094e4923451 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -3319,8 +3319,16 @@ static struct ext4_ext_path *ext4_split_extent_at(handle_t *handle,
>  			 * extent length and ext4_split_extent() split will the
>  			 * first half again.
>  			 */
> -			if (split_flag & EXT4_EXT_DATA_PARTIAL_VALID1)
> +			if (split_flag & EXT4_EXT_DATA_PARTIAL_VALID1) {
> +				/*
> +				 * Drop extent cache to prevent stale unwritten
> +				 * extents remaining after zeroing out.
> +				 */
> +				ext4_es_remove_extent(inode,
> +					le32_to_cpu(zero_ex.ee_block),
> +					ext4_ext_get_actual_len(&zero_ex));
>  				goto fix_extent_len;
> +			}
>  
>  			/* update the extent length and mark as initialized */
>  			ex->ee_len = cpu_to_le16(ee_len);
> -- 
> 2.46.1
>