Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with MMF_DISABLE_THP check

[Lance Yang]

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@xxxxxxxx> 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