linux-kernel - Re: [PATCH next v2 0/2] THP COW support for private executable file mmap

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Message-ID: <aVCnMI5gFLTB9UCe@casper.infradead.org>
Date: Sun, 28 Dec 2025 03:42:40 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Zhang Qilong <zhangqilong3@...wei.com>
Cc: akpm@...ux-foundation.org, david@...nel.org, lorenzo.stoakes@...cle.com,
	corbet@....net, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
	Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
	dev.jain@....com, baohua@...nel.org, lance.yang@...ux.dev,
	vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
	wangkefeng.wang@...wei.com, sunnanyong@...wei.com,
	linux-mm@...ck.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, lianux.mm@...il.com
Subject: Re: [PATCH next v2 0/2] THP COW support for private executable file
 mmap

On Fri, Dec 26, 2025 at 06:03:35PM +0800, Zhang Qilong wrote:
> The MySQL (Ver 8.0.25) test results on AMD are as follows:
> 
> -------------------------------------------------------------------
>                  | Exec mmap Rss(kB)  | Measured tpmC (NewOrders) |
> -----------------|--------------------|---------------------------|
>  base(page COW)  |       32868        |        339686             |
> -----------------|--------------------|---------------------------|
>  exec THP COW    |       43516        |        371324             |
> -------------------------------------------------------------------
> 
> The MySQL using exec THP COW consumes an additional 10648 kB of memory
> but achieves 9.3% performance improvement in the scenario of hotpatch.
> Additionally, another our internal program achieves approximately a 5%
> performance improvement as well.
> 
> As result, using exec THP COW will consume additional memory. The
> additional memory consumption may be negligible for the current system.
> It's necessary to balance the memory consumption with the performance
> impact.

I mean ... you say "negligible", I saay "32% extra".  9% performance
gain is certainly nothing to sneer at (and is consistent with measured
performance gains from using large folios for, eg, kernel compiles).
But wow, that's a lot of extra memory.  My feeling is that we shouldn't
add this functionality, but I'd welcome other opinions.