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

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Message-ID: <21c4788c-2516-2e47-5885-486c352c2b6e@linux.microsoft.com>
Date:   Thu, 17 Sep 2020 10:27:16 -0700
From:   Vijay Balakrishna <vijayb@...ux.microsoft.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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 9/17/2020 2:28 AM, Michal Hocko wrote:
> On Wed 16-09-20 23:39:39, Vijay Balakrishna wrote:
>> set_recommended_min_free_kbytes need to honor min_free_kbytes set by the
>> user.  Post start-of-day THP enable or memory hotplug operations can
>> lose user specified min_free_kbytes, in particular when it is higher than
>> calculated recommended value.
> 
> I was about to recommend a more detailed explanation when I have
> realized that this patch is not really needed after all. Unless I am
> missing something.
> 
> init_per_zone_wmark_min ignores the newly calculated min_free_kbytes if
> it is lower than user_min_free_kbytes. So calculated min_free_kbytes >=
> user_min_free_kbytes.
> 
> Except for value clamping when the value is reduced and this likely
> needs fixing. But set_recommended_min_free_kbytes should be fine.
> 

IIUC, after start-of-day if a user performs
- THP disable
- modifies min_free_bytes
- THP enable
above sequence currently wouldn't result in calling init_per_zone_wmark_min.

Thanks,
Vijay