Re: [PATCH 3/3] btrfs: don't hold an extra reference for redirtied buffers

[David Sterba <dsterba@suse.cz>]

On Mon, May 08, 2023 at 07:58:39AM -0700, Christoph Hellwig wrote:
> When btrfs_redirty_list_add redirties a buffer, it also acquires
> an extra reference that is released on transaction commit.  But
> this is not required as buffers that are dirty or under writeback
> are never freed (look for calls to extent_buffer_under_io())).
> 
> Remove the extra reference and the infrastructure used to drop it
> again.

I vaguely remember that the redirty list was need for zoned to avoid
some write pattern that disrupts the ordering, added in d3575156f662
("btrfs: zoned: redirty released extent buffers").

I'd appreciate more eyes on this patch, with the indirections and
writeback involved it's not clear to me that we don't need the list at
all. Pointing to extent_buffer_under_io() is a good start but the state
transitions of eb are complex so a more concrete example how it works
should be in the changelog.

For testing I'll add the series to misc-next, changelog update can be
done later.  Thanks.