Re: [PATCH 0/5] btrfs: remove folio ordered flag

[David Sterba <dsterba@suse.cz>]

On Thu, May 07, 2026 at 02:59:16PM +0930, Qu Wenruo wrote:
> [CHANGELOG]
> RFC->v1:
> - Replace the folio_test_ordered() inside extent_writepage_io()
>   Now do the check inside alloc_new_bio(), which is already doing the OE
>   search.
> 
>   Now we can detect dirty blocks without an OE at per-block level, much
>   better than the previous per-folio level checks, and without any extra
>   overhead.
> 
> Btrfs has a long history using an internal folio flag called ordered,
> which is to indicate if an fs block is covered by an ordered extent.
> 
> However this means we need to synchronize between ordered extents, which
> are managed by a per-inode ordered tree, and folio flag/subpage bitmap.
> 
> Furthermore with huge folio support, the ordered bitmap can be as large
> as 64 bytes (512 bits), which is not a small amount.
> 
> The series is going to remove folio ordered flag completely, along with
> the ordered subpage bitmap.
> 
> Most call sites of folio_test_ordered() are just inside ASSERT()s, so
> it's not too hard to remove them.
> 
> There are two call sites that utilizing *_folio_test_ordered():
> 
> - Inside extent_writepage_io()
>   The warning left by the legacy COW fixup mechanism.
>   The 1st patch is to introduce a more reliable way to detect dirty
>   blocks without an OE, other than checking the folio ordered flag, now
>   it's doing a per-block level check without introducing new overhead.
> 
> - Inside btrfs_invalidate_folio()
>   We use ordered flag to check if we can skip an ordered extent.
>   This is worked around by using the fact that we have waited for
>   writeback of the folio, so that endio should have already finished for
>   the writeback range. Then check dirty flags to determine if we can skip
>   the OE range.
> 
>   To get a reliable dirty flag for both sub-folio and full-folio cases, we
>   can not clear the folio dirty flag early, so the 2nd patch is
>   introduced to change the folio dirty flag clearing timing, then the
>   3rd patch can remove the folio_test_ordered() usage.
> 
> Then the 4th patch is to remove the remaining folio_test_ordered()
> usage, and finally we can remove the whole ordered flag/subpage bitmap
> completely.
> 
> I tried to hide the ordered flag/bitmap behind DEBUG, but unfortunately
> the subpage bitmap macros are not that easy to be tweaked to handle
> conditional ordered flags.
> 
> So the ordered flag/bitmap must be either there, or be completely gone.
> I hope enough test runs will cover the removed ASSERT()s.
> 
> Qu Wenruo (5):
>   btrfs: detect dirty blocks without an ordered extent more reliably
>   btrfs: unify folio dirty flag clearing
>   btrfs: use dirty flag to check if an ordered extent needs to be
>     truncated
>   btrfs: remove folio_test_ordered() usage
>   btrfs: remove folio ordered flag and subpage bitmap

Please add it to for-next.