patches/mm-page_alloc-prevent-reporting-pcp-batch-=-0.patch

From: Joshua Hahn <joshua.hahnjy@gmail.com>
Subject: mm/page_alloc: prevent reporting pcp->batch = 0
Date: Thu, 9 Oct 2025 12:29:31 -0700

zone_batchsize returns the appropriate value that should be used for
pcp->batch. If it finds a zone with less than 4096 pages or PAGE_SIZE >
1M, however, it leads to some incorrect math.

In the above case, we will get an intermediary value of 1, which is then
rounded down to the nearest power of two, and 1 is subtracted from it.
Since 1 is already a power of two, we will get batch = 1-1 = 0:

	batch = rounddown_pow_of_two(batch + batch/2) - 1;

A pcp->batch value of 0 is nonsensical. If this were actually set, then
functions like drain_zone_pages would become no-ops, since they could
only free 0 pages at a time.

Of the two callers of zone_batchsize, the one that is actually used to
set pcp->batch works around this by setting pcp->batch to the maximum
of 1 and zone_batchsize. However, the other caller, zone_pcp_init,
incorrectly prints out the batch size of the zone to be 0.

This is probably rare in a typical zone, but the DMA zone can often have
less than 4096 pages, which means it will print out "LIFO batch:0".

Before: [    0.001216]   DMA zone: 3998 pages, LIFO batch:0
After:  [    0.001210]   DMA zone: 3998 pages, LIFO batch:1

Instead of dealing with the error handling and the mismatch between the
reported and actual zone batchsize, just return 1 if the zone_batchsize
is 1 page or less before the rounding.

Link: https://lkml.kernel.org/r/20251009192933.3756712-3-joshua.hahnjy@gmail.com
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_alloc.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/mm/page_alloc.c~mm-page_alloc-prevent-reporting-pcp-batch-=-0
+++ a/mm/page_alloc.c
@@ -5866,8 +5866,8 @@ static int zone_batchsize(struct zone *z
 	 * and zone lock contention.
 	 */
 	batch = min(zone_managed_pages(zone) >> 12, SZ_256K / PAGE_SIZE);
-	if (batch < 1)
-		batch = 1;
+	if (batch <= 1)
+		return 1;
 
 	/*
 	 * Clamp the batch to a 2^n - 1 value. Having a power
@@ -6018,7 +6018,7 @@ static void zone_set_pageset_high_and_ba
 {
 	int new_high_min, new_high_max, new_batch;
 
-	new_batch = max(1, zone_batchsize(zone));
+	new_batch = zone_batchsize(zone);
 	if (percpu_pagelist_high_fraction) {
 		new_high_min = zone_highsize(zone, new_batch, cpu_online,
 					     percpu_pagelist_high_fraction);
_