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From: Lance Yang <ioworker0@gmail.com>
Date: Tue, 30 Jan 2024 11:08:10 +0800
Message-ID: <CAK1f24=2YE+BCYiizkqc8rmN8NaFv_Q6ZtE+4DiFK0PpcefOrQ@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with
 MMF_DISABLE_THP check
To: Michal Hocko <mhocko@suse.com>
Cc: akpm@linux-foundation.org, zokeefe@google.com, david@redhat.com, 
	songmuchun@bytedance.com, shy828301@gmail.com, peterx@redhat.com, 
	minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
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On Tue, Jan 30, 2024 at 10:12=E2=80=AFAM Lance Yang <ioworker0@gmail.com> w=
rote:
>
> 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.

In the Go standard library, there is no direct encapsulation
of the fork system call. Instead, fork and execve are
combined into one through syscall.ForkExec.

>
> BR,
> Lance
>
> On Tue, Jan 30, 2024 at 12:28=E2=80=AFAM Michal Hocko <mhocko@suse.com> w=
rote:
> >
> > 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