linux-kernel - Re: [PATCH] mm: always consider THP when adjusting min_free

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Date:   Thu, 6 Feb 2020 13:23:21 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Song Liu <songliubraving@...com>,
        "Kirill A.Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: always consider THP when adjusting min_free_kbytes

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