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Date: Wed, 14 Oct 2026 14:48:23 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Nimrod Oren <noren@nvidia.com>
Cc: David Hildenbrand <david@kernel.org>, Lorenzo Stoakes <ljs@kernel.org>,
 "Liam R. Howlett" <liam@infradead.org>, Vlastimil Babka
 <vbabka@kernel.org>, Mike Rapoport <rppt@kernel.org>, Suren Baghdasaryan
 <surenb@google.com>, Michal Hocko <mhocko@suse.com>, Jonathan Corbet
 <corbet@lwn.net>, Shuah Khan <skhan@linuxfoundation.org>, Randy Dunlap
 <rdunlap@infradead.org>, Zi Yan <ziy@nvidia.com>, Baolin Wang
 <baolin.wang@linux.alibaba.com>, Nico Pache <nico.pache@linux.dev>,
 "Ryan Roberts" <ryan.roberts@arm.com>, Dev Jain <dev.jain@arm.com>, Barry
 Song <baohua@kernel.org>, Lance Yang <lance.yang@linux.dev>, Usama Arif
 <usama.arif@linux.dev>, Kiryl Shutsemau <kas@kernel.org>, Brendan Jackman
 <brendan.jackman@linux.dev>, Johannes Weiner <hannes@cmpxchg.org>,
 "David Laight" <david.laight.linux@gmail.com>, <linux-mm@kvack.org>,
 <linux-doc@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: stop recalculating min_free_kbytes once set by
 userspace
Message-Id: <20261014144823.8c89da26c547fa95936a2fe5@linux-foundation.org>
In-Reply-To: <20261014140859.1006967-1-noren@nvidia.com>
References: <20261014140859.1006967-1-noren@nvidia.com>
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On Wed, 14 Oct 2026 17:08:59 +0300 Nimrod Oren <noren@nvidia.com> wrote:

> min_free_kbytes is recalculated on memory hotplug and on THP sysfs
> writes. The new value comes from calculate_min_free_kbytes() or, while
> THP is enabled, from set_recommended_min_free_kbytes(). A value written
> to the vm.min_free_kbytes sysctl is replaced whenever the new value is
> higher, so a lower value does not stick. This matters most when THP is
> enabled on configurations with large pageblocks, where the new value
> can be excessive [1].
> 
> Stop recalculating min_free_kbytes once a value has been written to the
> sysctl. Systems where the sysctl is never written keep the current
> behavior.

This problem lies somewhere in the range of "minor annoyance" to
"catastrophically horrible", but it isn't clear where in that range it
falls.

So can we please can we have a paragraph which explains the real-world
userspace-visible runtime effects of this problem?