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Date:   Thu, 6 Feb 2020 12:39:45 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Mike Kravetz <mike.kravetz@oracle.com>
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>
Subject: Re: [PATCH] mm: always consider THP when adjusting min_free_kbytes
Message-ID: <20200206203945.GZ8731@bombadil.infradead.org>
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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.