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Message-ID: <d535ebaf-1e5a-4f02-9d8b-5231dee317a1@kernel.org>
Date: Mon, 22 Jul 2024 11:28:42 +0200
From: "Vlastimil Babka (SUSE)" <vbabka@...nel.org>
To: "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>,
"linux-mm@...ck.org" <linux-mm@...ck.org>
Cc: "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"Yasunori Gotou (Fujitsu)" <y-goto@...itsu.com>,
David Hildenbrand <david@...hat.com>,
"Xingtao Yao (Fujitsu)" <yaoxt.fnst@...itsu.com>
Subject: Re: [PATCH v2] mm/page_alloc: Fix pcp->count race between
drain_pages_zone() vs __rmqueue_pcplist()
On 7/22/24 11:15 AM, Zhijian Li (Fujitsu) wrote:
> 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@...hat.com>
>>> Cc: Vlastimil Babka (SUSE) <vbabka@...nel.org>
>>> Reported-by: Yao Xingtao <yaoxt.fnst@...itsu.com>
>>> Signed-off-by: Li Zhijian <lizhijian@...itsu.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);
Yeah better than break. But I think you still should use
count = pcp->count;
...
count -= to_drain;
} while(count);
or you make one extra wasteful iteration to find to_drain is zero.
(assuming "it's sufficient to read pcp->count once with lock held", that I
agree with)
> }
>> 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
Yeah sorry I meant free_pcppages_bulk()
>> ...
>> spin_unlock(&pcp->lock);
>> if (count)
>> break;
>
> Thanks
> Zhijian
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