Re: [PATCH] btrfs: do compressed bio size roundup and zeroing in one go

[David Sterba <dsterba@suse.cz>]

On Fri, Feb 20, 2026 at 10:13:38AM +1030, Qu Wenruo wrote:
> Currently we zero out all the remaining bytes of the last folio of
> the compressed bio, then round the bio size to fs block boundary.
> 
> But that is done in two different functions, zero_last_folio() to zero
> the remaining bytes of the last folio, and round_up_last_block() to
> round up the bio to fs block boundary.
> 
> There are some minor problems:
> 
> - zero_last_folio() is zeroing ranges we won't submit
>   This is mostly affecting block size < page size cases, where we can
>   have a large folio (e.g. 64K), but the fs block size is only 4K.
> 
>   In that case, we may only want to submit the first 4K of the folio,
>   the remaining range won't matter, but we still zero them all.
> 
>   This causes unnecessary CPU usage just to zero out some bytes we won't
>   utilized.
> 
> - compressed_bio_last_folio() is called twice in two different functions
>   Which in theory we only need to call it once.
> 
> Enhance the situation by:
> 
> - Only zero out bytes up to the fs block boundary
>   Thus this will reduce some overhead for bs < ps cases.
> 
> - Move the folio_zero_range() call into round_up_last_block()
>   So that we can reuse the same folio returned by
>   compressed_bio_last_folio().
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Reviewed-by: David Sterba <dsterba@suse.com>