Re: [PATCH v2 2/3] dm zoned: do not reclaim a random zone into another random zone

[Damien Le Moal <dlemoal@kernel.org>]

On 2026/10/10 18:16, illofspeed 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 allow the random-zone fallback 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: illofspeed <illofspeed@gmail.com>
> ---
>  drivers/md/dm-zoned-reclaim.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-zoned-reclaim.c b/drivers/md/dm-zoned-reclaim.c
> index 315f0da99f3..2bcc163d209 100644
> --- a/drivers/md/dm-zoned-reclaim.c
> +++ b/drivers/md/dm-zoned-reclaim.c
> @@ -288,7 +288,14 @@ 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)) {
> +	/*
> +	 * Without a free sequential zone, moving a cache zone into a random
> +	 * zone of the zoned device still frees cache space. Moving a random
> +	 * zone into another random zone gains nothing, and when the target
> +	 * is idle it repeats forever. Only fall back for cache zones.

"fall back" to what? This comment is not clear.

> +	 */
> +	if (!szone && alloc_flags == DMZ_ALLOC_SEQ && dmz_nr_cache_zones(zmd) &&

Long line. Please split after "DMZ_ALLOC_SEQ &&".
Also, is the test for dmz_nr_cache_zones() needed? dmz_is_cache(dzone) will
never be true if there are no cache zones.

> +	    dmz_is_cache(dzone)) {
>  		alloc_flags = DMZ_ALLOC_RND;
>  		goto again;
>  	}


-- 
Damien Le Moal
Western Digital Research