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Date:   Thu, 17 Sep 2020 11:34:13 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Vijay Balakrishna <vijayb@linux.microsoft.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Song Liu <songliubraving@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Allen Pais <apais@microsoft.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [v4] mm: khugepaged: avoid overriding min_free_kbytes set by user
Message-ID: <20200917093413.GB29887@dhcp22.suse.cz>
References: <1600324779-6782-1-git-send-email-vijayb@linux.microsoft.com>
 <20200917092805.GA29887@dhcp22.suse.cz>
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On Thu 17-09-20 11:28:06, Michal Hocko wrote:
> On Wed 16-09-20 23:39:39, Vijay Balakrishna wrote:
> > set_recommended_min_free_kbytes need to honor min_free_kbytes set by the
> > user.  Post start-of-day THP enable or memory hotplug operations can
> > lose user specified min_free_kbytes, in particular when it is higher than
> > calculated recommended value.
> 
> I was about to recommend a more detailed explanation when I have
> realized that this patch is not really needed after all. Unless I am
> missing something.
> 
> init_per_zone_wmark_min ignores the newly calculated min_free_kbytes if
> it is lower than user_min_free_kbytes. So calculated min_free_kbytes >=
> user_min_free_kbytes.
> 
> Except for value clamping when the value is reduced and this likely
> needs fixing. But set_recommended_min_free_kbytes should be fine.

Something like
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fab5e97dc9ca..69731b19d9bf 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7875,9 +7875,9 @@ int __meminit init_per_zone_wmark_min(void)
 	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;
+			min_free_kbytes = max(128, user_min_free_kbytes);
 		if (min_free_kbytes > 262144)
-			min_free_kbytes = 262144;
+			min_free_kbytes = max(262144, user_min_free_kbytes);
 	} 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);
-- 
Michal Hocko
SUSE Labs