linux-kernel - Re: [PATCH] mm/rmap: optimize folio_move_anon

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Date: Tue, 14 May 2024 08:55:43 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: David Hildenbrand <david@...hat.com>,
 Chen Taotao <chentt10@...natelecom.cn>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/rmap: optimize folio_move_anon_rmap()



On 5/13/24 21:07, David Hildenbrand wrote:
> On 12.05.24 14:35, Chen Taotao wrote:
>> When a folio belongs exclusively to one process after a COW event,
>> folio_move_anon_rmap() always moves the folio into the anon_vma
>> belongs only to this process.
>>
>> However, if the folio already belongs to the anon_vma of the this
>> process, we don't need to move it again. In this case, we first
>> check if the folio already belongs to the anna_vma of the this
>> process, and only move it if it does not.
>>
>> The above changes may improve the performance of vm faults in some
>> scenarios, because the performance loss caused by WRITE_ONCE() is
>> much more than the performance loss caused by add a judgment.
> 
> Please proof that by real numbers. I don't think it will make a real difference, and we likely don't want that change.

Agreed, only scenarios when pre-condition check makes any sense is
if subsequent actions are expensive such as writing into registers
etc. But in this case both 'if' and 'WRITE_ONCE' statements are of
comparable cost, hence adding additional conditional check is only
going to increase the cost on average i.e when both gets executed.