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/
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
Reported-by: Yao Xingtao <yaoxt.fnst@xxxxxxxxxxx>
Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxx>
---
V2:
- Narrow down the scope of the spin_lock() to limit the draining latency. # Vlastimil and David
- In above scenario, it's sufficient to read pcp->count once with lock held, and it fully fixed
my issue[1] in thounds runs(It happened in more than 5% before).
RFC:
https://lore.kernel.org/linux-mm/20240716073929.843277-1-lizhijian@xxxxxxxxxxx/
---
mm/page_alloc.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9ecf99190ea2..5388a35c4e9c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2323,8 +2323,11 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
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);
+ int count;
+ spin_lock(&pcp->lock);
+ count = pcp->count;
+ spin_unlock(&pcp->lock);
while (count) {
int to_drain = min(count, pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX);
count -= to_drain;
--
2.29.2
