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

[David Hildenbrand <david@redhat.com>]

On 18.07.24 13:16, Vlastimil Babka (SUSE) wrote:
> On 7/16/24 9:39 AM, Li Zhijian wrote:
>> 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@fujitsu.com/
>>
>> Cc: David Hildenbrand <david@redhat.com>
>> Reported-by: Yao Xingtao <yaoxt.fnst@fujitsu.com>
>> Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
>> ---
>>   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);
> 
> This way seems to be partially going against the purpose of 55f77df7d715
> ("mm: page_alloc: control latency caused by zone PCP draining") - the zone
> lock hold time will still be limited by the batch, but not the pcp lock
> time. It should still be possible to relock between the iterations? To
> prevent the race I think the main part is determining pcp->count under the
> lock, but release/retake should still be ok if the pcp->count is reread
> after relocking.

Agreed, had the smame thing in mind when skimming over this patch.

@Li, with this patch the problems you have been seeing are fully 
resolved, correct?

-- 
Cheers,

David / dhildenb