The patch titled
Subject: mm/page_alloc: fix pcp->count race between drain_pages_zone() vs __rmqueue_pcplist()
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-page_alloc-fix-pcp-count-race-between-drain_pages_zone-vs-__rmqueue_pcplist.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page_alloc-fix-pcp-count-race-between-drain_pages_zone-vs-__rmqueue_pcplist.patch
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: Li Zhijian <lizhijian@xxxxxxxxxxx>
Subject: mm/page_alloc: fix pcp->count race between drain_pages_zone() vs __rmqueue_pcplist()
Date: Tue, 23 Jul 2024 14:44:28 +0800
It's expected that no page should be left in pcp_list after calling
zone_pcp_disable() in offline_pages(). Previously, it's observed that
offline_pages() gets stuck [1] due to some pages remaining in pcp_list.
Cause:
There is a race condition between drain_pages_zone() and __rmqueue_pcplist()
involving the pcp->count variable. See below scenario:
CPU0 CPU1
---------------- ---------------
spin_lock(&pcp->lock);
__rmqueue_pcplist() {
zone_pcp_disable() {
/* list is empty */
if (list_empty(list)) {
/* add pages to pcp_list */
alloced = rmqueue_bulk()
mutex_lock(&pcp_batch_high_lock)
...
__drain_all_pages() {
drain_pages_zone() {
/* read pcp->count, it's 0 here */
count = READ_ONCE(pcp->count)
/* 0 means nothing to drain */
/* update pcp->count */
pcp->count += alloced << order;
...
...
spin_unlock(&pcp->lock);
In this case, after calling zone_pcp_disable() though, there are still some
pages in pcp_list. And these pages in pcp_list are neither movable nor
isolated, offline_pages() gets stuck as a result.
Solution:
Expand the scope of the pcp->lock to also protect pcp->count in
drain_pages_zone(), to ensure no pages are left in the pcp list after
zone_pcp_disable()
[1] https://lore.kernel.org/linux-mm/6a07125f-e720-404c-b2f9-e55f3f166e85@xxxxxxxxxxx/
Link: https://lkml.kernel.org/r/20240723064428.1179519-1-lizhijian@xxxxxxxxxxx
Fixes: 4b23a68f9536 ("mm/page_alloc: protect PCP lists with a spinlock")
Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxx>
Reported-by: Yao Xingtao <yaoxt.fnst@xxxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)
--- a/mm/page_alloc.c~mm-page_alloc-fix-pcp-count-race-between-drain_pages_zone-vs-__rmqueue_pcplist
+++ a/mm/page_alloc.c
@@ -2343,16 +2343,20 @@ void drain_zone_pages(struct zone *zone,
static void drain_pages_zone(unsigned int cpu, struct zone *zone)
{
struct per_cpu_pages *pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
- int count = READ_ONCE(pcp->count);
-
- while (count) {
- int to_drain = min(count, pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX);
- count -= to_drain;
+ int count;
+ do {
spin_lock(&pcp->lock);
- free_pcppages_bulk(zone, to_drain, pcp, 0);
+ count = pcp->count;
+ if (count) {
+ int to_drain = min(count,
+ pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX);
+
+ free_pcppages_bulk(zone, to_drain, pcp, 0);
+ count -= to_drain;
+ }
spin_unlock(&pcp->lock);
- }
+ } while (count);
}
/*
_
Patches currently in -mm which might be from lizhijian@xxxxxxxxxxx are
mm-page_alloc-fix-pcp-count-race-between-drain_pages_zone-vs-__rmqueue_pcplist.patch
