Re: [PATCH v3 2/3] dm zoned: do not reclaim a random zone into another random zone

[Hannes Reinecke <hare@suse.de>]

On 10/11/26 7:54 PM, Kyrylo Biryuk wrote:
> dmz_reclaim_rnd_data() falls back to a random destination zone when no
> free sequential zone is available and the target has cache zones. When
> the target is idle and the cache zone list is empty,
> dmz_get_rnd_zone_for_reclaim() selects random zones of the zoned device
> as well, so a random zone is then copied into another random zone. That
> frees nothing: the next pass picks the next random zone and copies it
> again, for as long as the target stays idle.
> 
> Seen on a two-device target (256 GiB regular cache device plus a 27 TB
> host-managed drive) used as an md RAID5 member: after a full rebuild
> every chunk was mapped and no sequential zone was free, and the idle
> drive read and wrote about 72 MB/s each without pause for hours while
> the free zone counters in "dmsetup status" did not change. Reproduced on
> emulated host-managed drives (tcmu-runner ZBC handler): 340 MB/s of
> copying on an idle target, 0 MB/s with this patch.
> 
> Only switch to a random destination zone when the zone being reclaimed
> is a cache zone: moving it to the zoned device still frees cache space.
> 
> Fixes: c5c788595292 ("dm zoned: start reclaim with sequential zones")
> Assisted-by: LLM
> Signed-off-by: Kyrylo Biryuk <illofspeed@gmail.com>
> ---
>   drivers/md/dm-zoned-reclaim.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-zoned-reclaim.c b/drivers/md/dm-zoned-reclaim.c
> index 315f0da99f3..88d56d3c86d 100644
> --- a/drivers/md/dm-zoned-reclaim.c
> +++ b/drivers/md/dm-zoned-reclaim.c
> @@ -288,7 +288,13 @@ static int dmz_reclaim_rnd_data(struct dmz_reclaim *zrc, struct dm_zone *dzone)
>   again:
>   	szone = dmz_alloc_zone(zmd, zrc->dev_idx,
>   			       alloc_flags | DMZ_ALLOC_RECLAIM);
> -	if (!szone && alloc_flags == DMZ_ALLOC_SEQ && dmz_nr_cache_zones(zmd)) {
> +	/*
> +	 * If no free sequential zone is available, a cache zone can still be
> +	 * moved into a free random zone of the zoned device, which frees cache
> +	 * space. Do not do this for a random zone: moving it into another
> +	 * random zone frees nothing and, on an idle target, repeats forever.
> +	 */
> +	if (!szone && alloc_flags == DMZ_ALLOC_SEQ && dmz_is_cache(dzone)) {
>   		alloc_flags = DMZ_ALLOC_RND;
>   		goto again;
>   	}

Reviewed-by: Hannes Reinecke <hare@kernel.org>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
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