The patch titled Subject: Revert "mm/page_alloc.c: don't show protection in zone's ->lowmem_reserve[] for empty zone" has been added to the -mm mm-hotfixes-unstable branch. Its filename is revert-mm-page_allocc-dont-show-protection-in-zones-lowmem_reserve-for-empty-zone.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Gabriel Krisman Bertazi krisman@suse.de Subject: Revert "mm/page_alloc.c: don't show protection in zone's ->lowmem_reserve[] for empty zone" Date: Tue, 25 Feb 2025 22:22:58 -0500

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.

For reference, I collected vmstat with and without this patch on a freshly booted system running intensive randread io from an nvme for 5 minutes. I got:

rpm-6.12.0-slfo.1.2 -> pgscan_kswapd 5629543865 Patched -> pgscan_kswapd 33580844

33M scans is similar to what we had in kernels predating this patch. These numbers is fairly representative of the workload on this machine, as measured in several runs. So we are talking about a 2-order of magnitude increase.

Link: https://lkml.kernel.org/r/20250226032258.234099-1-krisman@suse.de 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 Acked-by: Michal Hocko mhocko@suse.com Cc: Mel Gorman mgorman@suse.de Cc: Baoquan He bhe@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/page_alloc.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/mm/page_alloc.c~revert-mm-page_allocc-dont-show-protection-in-zones-lowmem_reserve-for-empty-zone +++ a/mm/page_alloc.c @@ -5851,11 +5851,10 @@ static void setup_per_zone_lowmem_reserv

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; _

Patches currently in -mm which might be from krisman@suse.de are

revert-mm-page_allocc-dont-show-protection-in-zones-lowmem_reserve-for-empty-zone.patch