linux-kernel - Re: [PATCH] mm/page_alloc: skip THP-sized PCP list when allocating non-CMA THP-sized page

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Date: Mon, 17 Jun 2024 19:36:13 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Barry Song <21cnbao@...il.com>
Cc: yangge1116 <yangge1116@....com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, liuzixing@...on.cn,
 Johannes Weiner <hannes@...xchg.org>, Vlastimil Babka <vbabka@...e.cz>,
 Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH] mm/page_alloc: skip THP-sized PCP list when allocating
 non-CMA THP-sized page



On 2024/6/17 18:43, Barry Song wrote:
> On Thu, Jun 6, 2024 at 3:07 PM Baolin Wang
> <baolin.wang@...ux.alibaba.com> wrote:
>>
>>
>>
>> On 2024/6/4 20:36, yangge1116 wrote:
>>>
>>>
>>> 在 2024/6/4 下午8:01, Baolin Wang 写道:
>>>> Cc Johannes, Zi and Vlastimil.
>>>>
>>>> On 2024/6/4 17:14, yangge1116@....com wrote:
>>>>> From: yangge <yangge1116@....com>
>>>>>
>>>>> Since commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for
>>>>> THP-sized allocations") no longer differentiates the migration type
>>>>> of pages in THP-sized PCP list, it's possible to get a CMA page from
>>>>> the list, in some cases, it's not acceptable, for example, allocating
>>>>> a non-CMA page with PF_MEMALLOC_PIN flag returns a CMA page.
>>>>>
>>>>> The patch forbids allocating non-CMA THP-sized page from THP-sized
>>>>> PCP list to avoid the issue above.
>>>>>
>>>>> Fixes: 5d0a661d808f ("mm/page_alloc: use only one PCP list for
>>>>> THP-sized allocations")
>>>>> Signed-off-by: yangge <yangge1116@....com>
>>>>> ---
>>>>>    mm/page_alloc.c | 10 ++++++++++
>>>>>    1 file changed, 10 insertions(+)
>>>>>
>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>> index 2e22ce5..0bdf471 100644
>>>>> --- a/mm/page_alloc.c
>>>>> +++ b/mm/page_alloc.c
>>>>> @@ -2987,10 +2987,20 @@ struct page *rmqueue(struct zone
>>>>> *preferred_zone,
>>>>>        WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
>>>>>        if (likely(pcp_allowed_order(order))) {
>>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>>> +        if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & ALLOC_CMA ||
>>>>> +                        order != HPAGE_PMD_ORDER) {
>>>>
>>>> Seems you will also miss the non-CMA THP from the PCP, so I wonder if
>>>> we can add a migratetype comparison in __rmqueue_pcplist(), and if
>>>> it's not suitable, then fallback to buddy?
>>>
>>> Yes, we may miss some non-CMA THPs in the PCP. But, if add a migratetype
>>> comparison in __rmqueue_pcplist(), we may need to compare many times
>>> because of pcp batch.
>>
>> I mean we can only compare once, focusing on CMA pages.
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 3734fe7e67c0..960a3b5744d8 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2973,6 +2973,11 @@ struct page *__rmqueue_pcplist(struct zone *zone,
>> unsigned int order,
>>                   }
>>
>>                   page = list_first_entry(list, struct page, pcp_list);
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +               if (order == HPAGE_PMD_ORDER &&
>> !is_migrate_movable(migratetype) &&
>> +                   is_migrate_cma(get_pageblock_migratetype(page)))
>> +                       return NULL;
>> +#endif
> 
> This doesn't seem ideal either. It's possible that the PCP still has many
> non-CMA folios, but due to bad luck, the first entry is "always" CMA.
> In this case,
> allocations with is_migrate_movable(migratetype) == false will always lose the
> chance to use the PCP.   It also appears to incur a PCP spin lock/unlock.

Yes, just some ideas to to mitigate the issue...

> 
> I don't see an ideal solution unless we bring back the CMA PCP :-)

Tend to agree, and adding a CMA PCP seems the overhead can be acceptable?