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Subject: Re: [PATCH] mm: always consider THP when adjusting min_free_kbytes
From:   Khalid Aziz <khalid.aziz@oracle.com>
To:     Mike Kravetz <mike.kravetz@oracle.com>,
        David Rientjes <rientjes@google.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Song Liu <songliubraving@fb.com>,
        "Kirill A.Shutemov" <kirill.shutemov@linux.intel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>
Date:   Tue, 04 Feb 2020 16:37:30 -0700
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References: <20200204194156.61672-1-mike.kravetz@oracle.com>
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         <8cc18928-0b52-7c2e-fbc6-5952eb9b06ab@oracle.com>
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On Tue, 2020-02-04 at 13:42 -0800, Mike Kravetz wrote:
> On 2/4/20 12:33 PM, David Rientjes wrote:
> > 
> > So it looks like this is fixing an obvious correctness issue but
> > also now 
> > requires users to rewrite the sysctl if they want to decrease the
> > min 
> > watermark.
> 
> Moving the call to khugepaged_adjust_min_free_kbytes as described
> above
> would avoid the THP adjustment unless we were going to overwrite the
> user defined value.  Now, I am not sure overwriting the user defined
> value
> as is done today is actually the correct thing to do.
> 
> Thoughts?
> Perhaps we should never overwrite a user defined value?

We might need to override user defined value if it is too low but
overriding it silently is not quite right. We should print a warning
at least. On the other hand, a user setting min_free_kbytes should know
what they are doing and if they set it too low, they have been warned
in the sysctl documentation. I would say we never override user defined
value but print a warning if the value is too low and kernel would have
adjusted it if it were not for the user defined value.

--
Khalid