Re: [PATCH v2 0/5] btrfs: bs > ps support preparation

[David Sterba <dsterba@suse.cz>]

On Tue, Sep 02, 2025 at 06:32:11PM +0930, Qu Wenruo wrote:
> One of the blockage for bs > ps support is the conflicts between all the
> single-page bvec iterator/helpers (like memzero_bvec(),
> bio_for_each_segment() etc) and large folios (with highmem support).
> 
> For bs > ps support, all the folios will have a minimal order, so that
> each folio will cover at least one block. This saves the hassle of the
> fs to handle sub-block contents.
> 
> However for all those single-page bvec iterator/helpers, they can only
> handle a bvec that is no larger than a page.
> 
> To address the conflicting features, go a completely different way to
> handle a fs block:
> 
> - Use phys_addr_t to represent a block inside a bio
>   So we won't need to bother the sp bvec helpers, just pass a single
>   paddr around.
> 
> - Do proper highmem handling for checksum generation/verification
>   Now we will grab the folio from using the paddr, and make sure the
>   folio will cover at least one block starting at the paddr.
> 
>   If the folio is highmem, do proper per-page kmap_local_folio()/kunmap()
>   to handle highmem.
>   Otherwise do a full block csum calculation in one go.
> 
>   This should brings no extra overhead except the paddr->folio
>   conversion (which should be really tiny), as for systems without
>   HIGHMEM, folio_test_partial_kmap() will always return false, and the
>   HIGHMEM path will be optimized out by the compiler completely.
> 
>   Unfortunately I don't have a 32bit VM at hand to test.
> 
> - Introduce extra marcos to iterate blocks inside a bio
>   Two macros, btrfs_bio_for_each_block() which starts at the specified
>   bio_iter.
>   The other one, btrfs_bio_for_each_block_all() will go through all
>   blocks in the bio.
> 
>   Both returns a @paddr representing a block. Callers are either using
>   paddr based helper like
>   btrfs_calculate_block_csum()/btrfs_check_block_csum(), or RAID56 which
>   is already using paddr.
> 
>   For now it's only utilized by btrfs, bcachefs has a similar helper and
>   that's my inspiration.
> 
>   I hope one day it can be escalated to bio.h.
> 
> With all those preparation done, btrfs now can support basic file
> opeartions with bs > ps support, but still with quite some limits:
> 
> - No compression support
>   The compressed folios must be allocated using the minimal folio order.
>   As btrfs_calculate_block_csum() requires the minimal folio size.
> 
> - No RAID56 support
> - No scrub support
>   The same as compression, currently we're allocating the folios in page
>   size.
>   Although raid56 codes are now using the btrfs_bio_for_each_block*()
>   helpers, the underlying folio sizes still needs update.

Even with the limitations this is a good start. The changes look
reasonable, there are some minor things to consider but otherwise please
add it to for-next as you see fit. Regarding testing on 32bit it's
getting harder as the support in distros is fading out.