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Date: Mon, 31 Aug 2026 17:00:16 +0800
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Subject: Re: [PATCH v2] mm/khugepaged: cap min_free_kbytes recommendation at 1
 GiB
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To: Nimrod Oren <noren@nvidia.com>
Cc: "Liam R. Howlett" <liam@infradead.org>, Mike Rapoport <rppt@kernel.org>,
 Suren Baghdasaryan <surenb@google.com>, Vlastimil Babka <vbabka@kernel.org>,
 David Hildenbrand <david@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
 Michal Hocko <mhocko@suse.com>, 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>,
 Usama Arif <usama.arif@linux.dev>, Kiryl Shutsemau <kas@kernel.org>,
 Nirmoy Das <nirmoyd@nvidia.com>, Dragos Tatulea <dtatulea@nvidia.com>,
 linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
 Lorenzo Stoakes <ljs@kernel.org>, Andrew Morton <akpm@linux-foundation.org>
References: <20260831075635.2244437-1-noren@nvidia.com>
From: Lance Yang <lance.yang@linux.dev>
In-Reply-To: <20260831075635.2244437-1-noren@nvidia.com>
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On 2026/8/31 15:56, Nimrod Oren wrote:
> When THP is enabled, set_recommended_min_free_kbytes() may raise
> min_free_kbytes using a heuristic that scales with pageblock_nr_pages.
> With MIGRATE_PCPTYPES equal to 3, the formula accounts for 11
> pageblocks for every eligible populated zone before capping the result
> at 5% of low memory.
> 
> This is reasonable when a pageblock is 2 MiB, as on common 4 KiB page
> configurations, but scales poorly with larger base page sizes. With
> the default arm64 pageblock sizes, the contribution per eligible zone
> before the 5% cap is:
> 
>    4 KiB pages:   2 MiB pageblock,   22 MiB per zone
>   16 KiB pages:  32 MiB pageblock,  352 MiB per zone
>   64 KiB pages: 512 MiB pageblock,  5.5 GiB per zone
> 
> Consequently, min_free_kbytes can reach excessive and unwanted levels.
> 
> Add an absolute 1 GiB cap to the recommendation, in addition to the
> existing percentage cap. This bounds the automatic recommendation to a
> sane value on systems with large pageblocks while preserving existing
> behavior for typical systems with 2 MiB pageblocks.

Sorry, I haven't been following the previous discussion.

One nit: why 1 GiB? It feels a bit arbitrary to me ...