[PATCH RFC] mm/page_alloc: Fix pcp->count race between drain_pages_zone() vs __rmqueue_pcplist()

[Li Zhijian]

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(), ensuring no pages are left in the pcp list.

[1] https://lore.kernel.org/linux-mm/6a07125f-e720-404c-b2f9-e55f3f166e85@xxxxxxxxxxx/

Cc: David Hildenbrand <david@xxxxxxxxxx>
Reported-by: Yao Xingtao <yaoxt.fnst@xxxxxxxxxxx>
Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxx>
---
 mm/page_alloc.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9ecf99190ea2..1780df31d5f5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2323,16 +2323,17 @@ 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;
 	while (count) {
 		int to_drain = min(count, pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX);
 		count -= to_drain;
 
-		spin_lock(&pcp->lock);
 		free_pcppages_bulk(zone, to_drain, pcp, 0);
-		spin_unlock(&pcp->lock);
 	}
+	spin_unlock(&pcp->lock);
 }
 
 /*
-- 
2.29.2