Re: [PATCH AUTOSEL 6.14 099/642] btrfs: prevent inline data extents read from touching blocks beyond its range

[David Sterba <dsterba@suse.cz>]

On Mon, May 05, 2025 at 06:05:15PM -0400, Sasha Levin wrote:
> From: Qu Wenruo <wqu@suse.com>
> 
> [ Upstream commit 1a5b5668d711d3d1ef447446beab920826decec3 ]
> 
> Currently reading an inline data extent will zero out the remaining
> range in the page.
> 
> This is not yet causing problems even for block size < page size
> (subpage) cases because:
> 
> 1) An inline data extent always starts at file offset 0
>    Meaning at page read, we always read the inline extent first, before
>    any other blocks in the page. Then later blocks are properly read out
>    and re-fill the zeroed out ranges.
> 
> 2) Currently btrfs will read out the whole page if a buffered write is
>    not page aligned
>    So a page is either fully uptodate at buffered write time (covers the
>    whole page), or we will read out the whole page first.
>    Meaning there is nothing to lose for such an inline extent read.
> 
> But it's still not ideal:
> 
> - We're zeroing out the page twice
>   Once done by read_inline_extent()/uncompress_inline(), once done by
>   btrfs_do_readpage() for ranges beyond i_size.
> 
> - We're touching blocks that don't belong to the inline extent
>   In the incoming patches, we can have a partial uptodate folio, of
>   which some dirty blocks can exist while the page is not fully uptodate:
> 
>   The page size is 16K and block size is 4K:
> 
>   0         4K        8K        12K        16K
>   |         |         |/////////|          |
> 
>   And range [8K, 12K) is dirtied by a buffered write, the remaining
>   blocks are not uptodate.
> 
>   If range [0, 4K) contains an inline data extent, and we try to read
>   the whole page, the current behavior will overwrite range [8K, 12K)
>   with zero and cause data loss.
> 
> So to make the behavior more consistent and in preparation for future
> changes, limit the inline data extents read to only zero out the range
> inside the first block, not the whole page.
> 
> Reviewed-by: Filipe Manana <fdmanana@suse.com>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> Signed-off-by: David Sterba <dsterba@suse.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>

This is not a stable dependency and the patch is not fixing anything
but a preparation so this does not make much sense for stable backports,
please drop it. Thanks.