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

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Date:   Mon, 13 Nov 2023 20:59:37 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc:     ying.huang@...el.com, wangkefeng.wang@...wei.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 6:53 PM, 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.

Thanks for pointing out the part I missed.

I saw the migrate_pages() syscall is also using 
folio_estimated_sharers() to check if the folio is shared, and I wonder 
it will bring about any significant issues?

> I'll send WIP patches for one approach that can improve the situation 
> soonish.

Great. Look forward to seeing this:)