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Subject: Re: [v4] mm: khugepaged: avoid overriding min_free_kbytes set by user
To:     Michal Hocko <mhocko@suse.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Song Liu <songliubraving@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Allen Pais <apais@microsoft.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <1600324779-6782-1-git-send-email-vijayb@linux.microsoft.com>
 <20200917092805.GA29887@dhcp22.suse.cz>
From:   Vijay Balakrishna <vijayb@linux.microsoft.com>
Message-ID: <21c4788c-2516-2e47-5885-486c352c2b6e@linux.microsoft.com>
Date:   Thu, 17 Sep 2020 10:27:16 -0700
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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