Re: [PATCH 0/2] btrfs: remove COW fixup and checked folio flag

[David Sterba <dsterba@suse.cz>]

On Wed, Apr 08, 2026 at 01:55:30PM +0930, Qu Wenruo wrote:
> For experimental builds we're already rejecting dirty folios which
> don't have ordered flags since v6.15.
> 
> Unfortunately we're not yet removing that COW fixup machanism for
> non-experimental builds even at v7.0, as there are still some error
> corner cases that can lead to dirty folios without ordered flags.
> 
> After those years of testing, we're sure we won't need COW fixup for
> regular cases. Although I want to remove the COW fixup mechanism after
> all known reproducible cases are fixed, I run out of ideas.

I don't know if I understand this correctly, are there known bugs that
the fixup can catch, but there are not known fixes/root causes?

> Furthermore for the future of huge folios (order 9, 2M page on 4K page
> size systems), we can not afford the extra bitmap for a huge folio.
> In that case, a huge folio will need 64 bytes per bitmap, which is no
> longer a small amount.
> 
> I believe it's time to remove the COW fixup mechanism even for
> non-experimental builds, along with the checked folio flags.

Yes please, it has served us well and caused some scars but now it's
dead code.