linux-kernel - Re: [RFC PATCH] mm: support large folio numa balancing

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Date:   Mon, 13 Nov 2023 21:01:23 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>,
        David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc:     ying.huang@...el.com, willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: Re: [RFC PATCH] mm: support large folio numa balancing



On 11/13/2023 8:10 PM, Kefeng Wang wrote:
> 
> 
> On 2023/11/13 18:53, David Hildenbrand wrote:
>> On 13.11.23 11:45, Baolin Wang wrote:
>>> Currently, the file pages already support large folio, and supporting 
>>> for
>>> anonymous pages is also under discussion[1]. Moreover, the numa 
>>> balancing
>>> code are converted to use a folio by previous thread[2], and the 
>>> migrate_pages
>>> function also already supports the large folio migration.
>>>
>>> So now I did not see any reason to continue restricting NUMA 
>>> balancing for
>>> large folio.
>>
>> I recall John wanted to look into that. CCing him.
>>
>> I'll note that the "head page mapcount" heuristic to detect sharers will
>> now strike on the PTE path and make us believe that a large folios is
>> exclusive, although it isn't.
>>
>> As spelled out in the commit you are referencing:
>>
>> commit 6695cf68b15c215d33b8add64c33e01e3cbe236c
>> Author: Kefeng Wang <wangkefeng.wang@...wei.com>
>> Date:   Thu Sep 21 15:44:14 2023 +0800
>>
>>      mm: memory: use a folio in do_numa_page()
>>      Numa balancing only try to migrate non-compound page in 
>> do_numa_page(),
>>      use a folio in it to save several compound_head calls, note we use
>>      folio_estimated_sharers(), it is enough to check the folio 
>> sharers since
>>      only normal page is handled, if large folio numa balancing is 
>> supported, a
>>      precise folio sharers check would be used, no functional change 
>> intended.
>>
>>
>> I'll send WIP patches for one approach that can improve the situation 
>> soonish.
> 
> When convert numa balance to use folio, I make similar change, it works
> with large anon folio(test with v5), but David's precise folio sharers
> should be merged firstly, also if a large folio shared by many process,
> we maybe split it, don't sure about it, this need some evaluation.

IIUC, numa balancing will not split the large folio.