linux-kernel - Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

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Date: Thu, 4 Apr 2024 16:01:18 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org
Cc: mgorman@...hsingularity.net, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

On 4/4/24 2:19 PM, Baolin Wang wrote:
> 
> 
> On 2024/4/4 18:03, Vlastimil Babka wrote:
>> On 4/3/24 3:47 PM, Baolin Wang wrote:
>>> Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
>>> on the per-cpu lists") extends the PCP allocator to store THP pages, and
>>> it determines whether to cache THP pags in PCP by comparing with pageblock_order.
>>> But the pageblock_order is not always equal to THP order, it might also
>>> be MAX_PAGE_ORDER, which could prevent PCP from caching THP pages.
>>>
>>> Therefore, using HPAGE_PMD_ORDER instead to determine the need for caching
>>> THP for PCP can fix this issue
>>>
>>> Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
>>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> 
>> IIUC this happens with CONFIG_HUGETLB_PAGE disabled because HUGETLBFS is
>> disabled? But THPs are still enabled? I think there might be more of THP
> 
> Right, and seems the Powerpc arch will set pageblock_order via 
> set_pageblock_order() when the huge page sizes are variable (not sure if 
> this is always equal to THP order).
> 
> Moreover, it still does not make sense to use pageblock_order to 
> indicate a THP page, especially when we already have HPAGE_PMD_ORDER to 
> represent THP.
> 
>> working suboptimally in that case with pageblock_order being larger
>> (MAX_PAGE_ORDER).
>> 
>> In other words, should be rather make pageblock_order itself defined as
>> 
>>   min_t(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)
>> 
>> in case with !CONFIG_HUGETLB_PAGE but THP enabled.
> 
> Yes, this makes sense to me (I wonder why this wasn't done before?). I 
> can create a seperate patch to do this, what do you think? Thanks.

It probably wasn't anticipated that hugetlbfs would be disabled and THP
enabled. If you can create such patch, great!