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

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Date: Tue, 4 Jun 2024 20:36:00 +0800
From: yangge1116 <yangge1116@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org
Cc: 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



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.