linux-kernel - Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with MMF_DISABLE

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Date: Tue, 30 Jan 2024 10:12:19 +0800
From: Lance Yang <ioworker0@...il.com>
To: Michal Hocko <mhocko@...e.com>
Cc: akpm@...ux-foundation.org, zokeefe@...gle.com, david@...hat.com, 
	songmuchun@...edance.com, shy828301@...il.com, peterx@...hat.com, 
	minchan@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with
 MMF_DISABLE_THP check

Hey Michal,

Thanks for taking time to review!

On some servers within our company, we deploy a
daemon responsible for monitoring and updating
local applications. Some applications prefer not to
use THP, so the daemon calls prctl to disable THP
before fork/exec. Conversely, for other applications,
the daemon calls prctl to enable THP before fork/exec.

Ideally, the daemon should invoke prctl after the fork,
but its current implementation follows the described
approach.

BR,
Lance

On Tue, Jan 30, 2024 at 12:28 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Mon 29-01-24 13:45:51, Lance Yang wrote:
> > khugepaged scans the entire address space in the
> > background for each given mm, looking for
> > opportunities to merge sequences of basic pages
> > into huge pages. However, when an mm is inserted
> > to the mm_slots list, and the MMF_DISABLE_THP flag
> > is set later, this scanning process becomes
> > unnecessary for that mm and can be skipped to avoid
> > redundant operations, especially in scenarios with
> > a large address space.
>
> Is this a real problem? I thought that the prctl is called
> on the parent before fork/exec. Or are you aware of any
> applications which do call prctl late enough that the race
> would be actually observable?
> --
> Michal Hocko
> SUSE Labs