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

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Date:   Tue, 4 Feb 2020 13:42:43 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     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 2/4/20 12:33 PM, David Rientjes wrote:
> On Tue, 4 Feb 2020, Mike Kravetz wrote:
> 
> Hmm, if khugepaged_adjust_min_free_kbytes() increases min_free_kbytes for 
> thp, then the user has no ability to override this increase by using 
> vm.min_free_kbytes?
> 
> IIUC, with this change, it looks like memory hotplug events properly 
> increase min_free_kbytes for thp optimization but also doesn't respect a 
> previous user-defined value?

Good catch.

We should only call khugepaged_adjust_min_free_kbytes from the 'true'
block of this if statement in init_per_zone_wmark_min.

	if (new_min_free_kbytes > user_min_free_kbytes) {
		min_free_kbytes = new_min_free_kbytes;
		if (min_free_kbytes < 128)
			min_free_kbytes = 128;
		if (min_free_kbytes > 65536)
			min_free_kbytes = 65536;
	} else {
		pr_warn("min_free_kbytes is not updated to %d because user defined value %d is preferred\n",
				new_min_free_kbytes, user_min_free_kbytes);
	}

In the existing code, a hotplug event will cause min_free_kbytes to overwrite
the user defined value if the new value is greater.  However, you will get
the warning message if the user defined value is greater.  I am not sure if
this is the 'desired/expected' behavior?  We print a warning if the user value
takes precedence over our calculated value.  However, we do not print a message
if we overwrite the user defined value.  That doesn't seem right!

> 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?
-- 
Mike Kravetz