Re: [PATCH] Revert "mm/page_alloc.c: don't show protection in zone's ->lowmem_reserve[] for empty zone"

[Vlastimil Babka <vbabka@suse.cz>]

On 2/26/25 4:22 AM, Gabriel Krisman Bertazi wrote:
> Commit 96a5c186efff ("mm/page_alloc.c: don't show protection in zone's
> ->lowmem_reserve[] for empty zone") removes the protection of lower
> zones from allocations targeting memory-less high zones.  This had an
> unintended impact on the pattern of reclaims because it makes the
> high-zone-targeted allocation more likely to succeed in lower zones,
> which adds pressure to said zones.  I.e, the following corresponding
> checks in zone_watermark_ok/zone_watermark_fast are less likely to
> trigger:
> 
>         if (free_pages <= min + z->lowmem_reserve[highest_zoneidx])
>                 return false;
> 
> As a result, we are observing an increase in reclaim and kswapd scans,
> due to the increased pressure.  This was initially observed as increased
> latency in filesystem operations when benchmarking with fio on a machine
> with some memory-less zones, but it has since been associated with
> increased contention in locks related to memory reclaim.  By reverting
> this patch, the original performance was recovered on that machine.
> 
> The original commit was introduced as a clarification of the
> /proc/zoneinfo output, so it doesn't seem there are usecases depending
> on it, making the revert a simple solution.
> 
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Baoquan He <bhe@redhat.com>
> Fixes: 96a5c186efff ("mm/page_alloc.c: don't show protection in zone's ->lowmem_reserve[] for empty zone")
> Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_alloc.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 579789600a3c..fe986e6de7a0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5849,11 +5849,10 @@ static void setup_per_zone_lowmem_reserve(void)
>  
>  			for (j = i + 1; j < MAX_NR_ZONES; j++) {
>  				struct zone *upper_zone = &pgdat->node_zones[j];
> -				bool empty = !zone_managed_pages(upper_zone);
>  
>  				managed_pages += zone_managed_pages(upper_zone);
>  
> -				if (clear || empty)
> +				if (clear)
>  					zone->lowmem_reserve[j] = 0;
>  				else
>  					zone->lowmem_reserve[j] = managed_pages / ratio;