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

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Date:   Mon, 13 Nov 2023 15:49:24 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.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 13.11.23 13:59, Baolin Wang wrote:
> 
> 
> 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?

It's now used all over the place, in some places for making manual 
decisions (e.g., MADV_PAGEOUT works although it shouldn't) and more and 
more automatic places (e.g., the system ends up migrating a folio 
although it shouldn't). The nasty thing about it is that it doesn't give 
you "certainly exclusive" vs. "maybe shared" but "maybe exclusive" vs. 
"certainly shared".

IIUC, the side effect could be that we migrate folios because we assume 
they are exclusive even though they are actually shared. Right now, it's 
sufficient to not have the first page of the folio mapped anymore for 
that to happen.

Anyhow, it's worth mentioning that in the commit message as long as we 
have no better solution for that. For many cases it might be just tolerable.

> 
>> I'll send WIP patches for one approach that can improve the situation
>> soonish.
> 
> Great. Look forward to seeing this:)

I'm still trying to evaluate the performance hit of the additional 
tracking ... turns out there is no such thing as free food ;)

-- 
Cheers,

David / dhildenb