Re: [PATCH] btrfs: shrink the size of btrfs_bio

[David Sterba <dsterba@suse.cz>]

On Tue, Dec 09, 2025 at 07:55:03AM +1030, Qu Wenruo wrote:
> This is done by:
> 
> - Shrink the size of btrfs_bio::mirror_num
>   From 32 bits unsigned int to u16.
> 
>   Normally btrfs mirror number is either 0 (all profiles), 1 (all
>   profiles), 2 (DUP/RAID1/RAID10/RAID5), 3 (RAID1C3) or 4 (RAID1C4).
> 
>   But for RAID6 the mirror number can go as large as the number of
>   devices of that chunk.
> 
>   Currently the limit for number of devices for a data chunk is
>   BTRFS_MAX_DEVS(), which is around 500 for the default 16K nodesize.
>   And if going the max 64K nodesize, we can have a little over 2000
>   devices for a chunk.
> 
>   Although I'd argue it's way overkilled, we don't reject such cases yet
>   thus u8 is not going to cut it, and have to use u16 (max out at 64K).
> 
> - Use bit fields for boolean members
>   Although it's not always safe for racy call sites, those members are
>   safe.
> 
>   * csum_search_commit_root
>   * is_scrub
>     Those two are set immediately after bbio allocation and no more
>     writes after allocation, thus they are very safe.
> 
>   * async_csum
>   * can_use_append
>     Those two are set for each split range, and after that there is no
>     writes into those two members in different threads, thus they are
>     also safe.
> 
>   And there are spaces for 4 more bits before increasing the size of
>   btrfs_bio again, which should be future proof enough.
> 
> - Reorder the structure members
>   Now we always put the largest member first (after the huge 120 bytes
>   union), making it easier to fill any holes.
> 
> This reduce the size of btrfs_bio by 8 bytes, from 312 bytes to 304 bytes.
> 
> Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
> Changelog:
> v2:
> - Change mirror number from u8 to u16
>   As for RAID6 we can have cases (that's beyond 255 devices in a RAID6
>   chunk) where u8 is not large enough.
>   Thankfully u16 is large enough for the max number of devices possible
>   for a RAID6 chunk.
> 
>   And we have exactly a one-byte hole the in structure, and expanding
>   the widith of @mirror_num will not increase the size of btrfs_bio.

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