Re: [PATCH v2 0/3] btrfs: migrate IO path to folios

[David Sterba <dsterba@suse.cz>]

On Tue, Dec 12, 2023 at 12:58:35PM +1030, Qu Wenruo wrote:
> [CHANGELOG]
> v2:
> - Fix a PAGE_SHIFT usage in the 3rd patch on the data read path
>   I know this won't be touched any time soon, but considering it's
>   really one patch away from enabling higher order folios for metadata,
>   let's make the cleanup closer to perfection.
> 
> 
> One critical problem I hit the most during my initial higher order
> folios tests are, incorrect access to the pages which conflicts with the
> page flag policy.
> 
> Since folio flags are only set to certain pages according to their
> policies (PF_ANY, PF_HEAD, PF_ONLY_HEAD, PF_NO_TAIL, PF_NO_COMPOUND and
> the most weird on PF_SECOND), setting page flags violating the policy
> would immedate lead to VM_BUG_ON().
> 
> Thus no matter if we go compound page or folio, we can not go the
> page-by-page iteration helpers that easily.
> One of the hot spots which can lead to VM_BUG_ON()s are the endio
> helpers.
> 
> So this patch would:
> 
> - Make metadata set/get helpers to utilize folio interfaces
> 
> - Make subpage code to accept folios directly
>   This is to avoid btrfs_page_*() helpers to accept page pointers, which
>   is another hot spot which uses page pointer a lot.
> 
> - Migrate btrfs bio endio functions to utilize bio_for_each_folio_all()
>   This completely removes the ability to direct access page pointers.
>   Although we still need some extra folio_page(folio, 0) calls to keep
>   compatible with existing helpers.
> 
>   And since we're here, also fix the choas of btrfs endio functions'
>   naming scheme, now it would always be:
>     end_bbio_<target>_(read|write)
>   The <target> can be:
> 
>   - data
>     For non-compressed and non-encoeded operations
> 
>   - compressed
>     For compressed IO and encoded IO.
> 
>   - meta
> 
>   And since compressed IO path is utilizing unmapped pages (pages
>   without an address_space), thus they don't touch the page flags.
>   This makes compressed IO path a very good test bed for the initial
>   introduction of higher order folio.
> 
> Qu Wenruo (3):
>   btrfs: migrate get_eb_page_index() and get_eb_offset_in_page() to
>     folios
>   btrfs: migrate subpage code to folio interfaces
>   btrfs: migrate various btrfs end io functions to folios

Added to misc-next, thanks.