Re: [PATCH v2] mm/page_alloc: Fix pcp->count race between drain_pages_zone() vs __rmqueue_pcplist()

["Zhijian Li (Fujitsu)" <lizhijian@fujitsu.com>]

Hi David

Thanks for you quickly reply.


On 22/07/2024 14:44, Vlastimil Babka (SUSE) wrote:
> On 7/22/24 4:10 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(), 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@fujitsu.com/
>>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Vlastimil Babka (SUSE) <vbabka@kernel.org>
>> Reported-by: Yao Xingtao <yaoxt.fnst@fujitsu.com>
>> Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
> 
> Can we find a breaking commit for Fixes: ?

I haven't confirmed the FBC because my reproducer is not fit to run in the old kernel for some reasons.
but I noticed it didn't read the count without lock held since below commit

4b23a68f9536 mm/page_alloc: protect PCP lists with a spinlock
  


> 
>> ---
>> 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).
> 
> That should be ok indeed, but...
> 
>> RFC:
>>      https://lore.kernel.org/linux-mm/20240716073929.843277-1-lizhijian@fujitsu.com/
>> ---
>>   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;
> 
> It's wasteful to do a lock/unlock cycle just to read the count.

How about,

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, to_drain;
                                                                                                    
         do {
                 spin_lock(&pcp->lock);
                 to_drain = min(pcp->count, pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX);
                 free_pcppages_bulk(zone, to_drain, pcp, 0);
                 spin_unlock(&pcp->lock);
         } while (to_drain);
}


> It could
> rather look something like this:
> 

Sorry, I don't follow your code...

> while (true)
>      spin_lock(&pcp->lock);
>      count = pcp->count;
>      ...
>      count -= to_drain;
>      if (to_drain)
>          drain_zone_pages(...)

Which subroutine does this code belong to, why it involves drain_zone_pages

>      ...
>      spin_unlock(&pcp->lock);
>      if (count)
>           break;

Thanks
Zhijian