linux-kernel - Re: [v4] mm: khugepaged: avoid overriding min_free

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Message-ID: <20200922070726.dlw24lf3wd3p2ias@black.fi.intel.com>
Date:   Tue, 22 Sep 2020 10:07:26 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Vijay Balakrishna <vijayb@...ux.microsoft.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Song Liu <songliubraving@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Allen Pais <apais@...rosoft.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [v4] mm: khugepaged: avoid overriding min_free_kbytes set by user

On Mon, Sep 21, 2020 at 12:07:23PM -0700, Vijay Balakrishna wrote:
> > 
> > I would recommend reposting the patch which adds heuristic for THP (if
> > THP is enabled) into the hotplug path, arguing with the consistency and
> > surprising results when adding memory decreases the value.
> 
> I hope my reposted patch
> ([v3 1/2] mm: khugepaged: recalculate min_free_kbytes after memory hotplug
> as expected by khugepaged)
> change log is ok:
> 
> When memory is hotplug added or removed the min_free_kbytes must be
> recalculated based on what is expected by khugepaged.  Currently
> after hotplug, min_free_kbytes will be set to a lower default and higher
> default set when THP enabled is lost.  This change restores min_free_kbytes
> as expected for THP consumers.

Any scenario when hotremove would result in changing min_free_kbytes?

> > Your initial
> > problem is in sizing as mentioned in other email thread and you should
> > be investigating more but this inconsistency might really come as a
> > surprise.
> > 
> > All that if Kirill is reconsidering his initial position of course.
> > 
> 
> Kirill, can you comment or share your opinion?

Looking again, never decreasing min_free_kbytes is the most reasonable
policy. Sorry for the noise.

But I would like to see a scenario when hotremov will end up changing
min_free_kbytes. It's not obvious to me.

-- 
 Kirill A. Shutemov