linux-kernel - Re: [RFC 2/2] khugepaged: use upgrade_read() to optimize collapse_huge

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Message-ID: <Zw-pK_4wCvJHKfSi@casper.infradead.org>
Date: Wed, 16 Oct 2024 12:53:15 +0100
From: Matthew Wilcox <willy@...radead.org>
To: lizhe.67@...edance.com
Cc: peterz@...radead.org, mingo@...hat.com, will@...nel.org,
	longman@...hat.com, boqun.feng@...il.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC 2/2] khugepaged: use upgrade_read() to optimize
 collapse_huge_page

On Wed, Oct 16, 2024 at 12:36:00PM +0800, lizhe.67@...edance.com wrote:
> From: Li Zhe <lizhe.67@...edance.com>
> 
> In function collapse_huge_page(), we drop mmap read lock and get
> mmap write lock to prevent most accesses to pagetables. There is
> a small time window to allow other tasks to acquire the mmap lock.
> With the use of upgrade_read(), we don't need to check vma and pmd
> again in most cases.

This is clearly a performance optimisation.  So you must have some
numebrs that justify this, please include them.