Re: [PATCH] btrfs: zoned: allow disabling of zone auto relcaim

[David Sterba <dsterba@suse.cz>]

On Mon, Aug 09, 2021 at 07:50:59AM +0000, Naohiro Aota wrote:
> On Fri, Aug 06, 2021 at 06:27:04PM +0900, Johannes Thumshirn wrote:
> > Automatically reclaiming dirty zones might not always be desired for all
> > workloads, especially as there are currently still some rough edges with
> > the relocation code on zoned filesystems.
> > 
> > Allow disabling zone auto reclaim on a per filesystem basis.
> > 
> > Cc: Naohiro Aota <naohiro.aota@wdc.com>
> > Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> >
> > ---
> >  fs/btrfs/free-space-cache.c | 3 ++-
> >  fs/btrfs/sysfs.c            | 5 ++++-
> >  2 files changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
> > index 8eeb65278ac0..933e9de37802 100644
> > --- a/fs/btrfs/free-space-cache.c
> > +++ b/fs/btrfs/free-space-cache.c
> > @@ -2567,7 +2567,8 @@ static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
> >  	/* All the region is now unusable. Mark it as unused and reclaim */
> >  	if (block_group->zone_unusable == block_group->length) {
> >  		btrfs_mark_bg_unused(block_group);
> > -	} else if (block_group->zone_unusable >=
> > +	} else if (fs_info->bg_reclaim_threshold &&
> > +		   block_group->zone_unusable >=
> >  		   div_factor_fine(block_group->length,
> >  				   fs_info->bg_reclaim_threshold)) {
> 
> nit: can this race with btrfs_bg_reclaim_threshold_store()'s
> bg_reclaim_threshold assignment? Then, we can end up doing
> div_factor_fine(block_group->length, 0)?

Good point, this should be READ_ONCE and then check if it's 0.