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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>
 <21c4788c-2516-2e47-5885-486c352c2b6e@linux.microsoft.com>
 <20200917175217.GD29887@dhcp22.suse.cz>
From:   Vijay Balakrishna <vijayb@linux.microsoft.com>
Message-ID: <f2231b44-6d14-fb63-43cd-43aaad02bef1@linux.microsoft.com>
Date:   Thu, 17 Sep 2020 11:16:55 -0700
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On 9/17/2020 10:52 AM, Michal Hocko wrote:
> On Thu 17-09-20 10:27:16, Vijay Balakrishna wrote:
>>
>>
>> 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.
> 
> I will not, but why do you think this matters? All we should care about
> is that auto-tuning shouldn't reduce user provided value [1] and that
> the memory hotplug should be consistent with the boot time heuristic.
> init_per_zone_wmark_min should make sure that the user value is not
> reduced and thp heuristic makes sure it will not reduce this value.
> So the property should be transitive with the existing code (modulo the
> problem I have highlighted).
> 
> [1] one could argue that it shouldn't even increase the value strictly
> speaking because an admin might have a very good reason to decrease the
> value but this has never been the semantic and changing it now might be
> problematic
> 

I made an attempt to address Kirill A. Shutemov's comment.  And incrased 
min_free_kbytes to see the issue in my testing and attempted a fix.  I'm 
ok leaving as it is.  Do not want introduce any changes that may cause 
regression.

Thanks,
Vijay