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Subject: Re: [PATCH] mm: always consider THP when adjusting min_free_kbytes
To:     Matthew Wilcox <willy@infradead.org>
Cc:     David Rientjes <rientjes@google.com>, 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>
References: <20200204194156.61672-1-mike.kravetz@oracle.com>
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From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <5e7800f2-3df3-a597-c164-5537b7f66417@oracle.com>
Date:   Thu, 6 Feb 2020 13:23:21 -0800
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On 2/6/20 12:39 PM, Matthew Wilcox wrote:
> On Wed, Feb 05, 2020 at 05:36:44PM -0800, Mike Kravetz wrote:
>> The value of min_free_kbytes is calculated in two routines:
>> 1) init_per_zone_wmark_min based on available memory
>> 2) set_recommended_min_free_kbytes may reserve extra space for
>>    THP allocations
>>
>> In both of these routines, a user defined min_free_kbytes value will
>> be overwritten if the value calculated in the code is larger. No message
>> is logged if the user value is overwritten.
>>
>> Change code to never overwrite user defined value.  However, do log a
>> message (once per value) showing the value calculated in code.
> 
> But what if the user set min_free_kbytes to, say, half of system memory,
> and then hot-unplugs three quarters of their memory?  I think the kernel
> should protect itself against such foolishness.

I'm not sure what we should set it to in this case.  Previously you said,

>> I'm reluctant to suggest we do a more complex adjustment of the value
>> (eg figure out what the adjustment would have been, then apply some
>> fraction of that adjustment to keep the ratios in proportion) because
>> we don't really know why they adjusted it.

So, I suspect you would suggest setting it to the default computed value?
But then, when do we start adjusting?  What if they only remove a small
amount of memory?  And, then add the same amount back in?

BTW - In the above scenario existing code would not change min_free_kbytes
because the user defined value is greater than value computed in code.
-- 
Mike Kravetz