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Thu, 23 Apr 2026 14:08:01 -0400 (EDT) Date: Thu, 23 Apr 2026 19:08:00 +0100 From: Kiryl Shutsemau To: Peter Xu Cc: "David Hildenbrand (Arm)" , Andrew Morton , Lorenzo Stoakes , Mike Rapoport , Suren Baghdasaryan , Vlastimil Babka , "Liam R . Howlett" , Zi Yan , Jonathan Corbet , Shuah Khan , Sean Christopherson , Paolo Bonzini , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [RFC, PATCH 00/12] userfaultfd: working set tracking for VM guest memory Message-ID: References: <1a499781-1115-44bc-adbf-2ac3769354ca@kernel.org> <4c635703-3d8d-4cfa-bb98-7f6f5fcbe547@kernel.org> <34f75083-29a3-4860-8a6e-94551d37ac6a@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: ECFF180010 X-Stat-Signature: s78g1dwb3s59iqmpz9bnugmpn6jtwnaw X-Rspam-User: X-HE-Tag: 1776967684-197724 X-HE-Meta: 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 Tm/sZbby VJpp1QJSTu1OWe8y7z5OA9efbq15QJK4PEDcHIKsjrZQP1mJgW/5czW+i3+QvFWpnhtxId5qbdaAMaSsvrZeHV+E2n0IUkGQ3fqkysFG6xe49yusOlI8KtqchkRMQJa1y4kDmcEwXBa3IOqyKfM78TTg92gCeRDiZ6EhckqFJki12UELYtrYYO6EH0xQ07Nl3RNPQLH/uPY0V64jzSY1OMxsrh1VyaxBNkKT2V62OgMqzHz1fpwVpPTNkTn8QmlZfIBrnQGeBlP/78Wyq8TVDUkyb/aNA2F1uW8NqDTQ58mdY7yw= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Apr 23, 2026 at 10:50:06AM -0400, Peter Xu wrote: > Hello, Kiryl, > > On Thu, Apr 23, 2026 at 03:27:11PM +0100, Kiryl Shutsemau wrote: > > The patchet is pretty good shape in my eyes and will probably drop RFC > > tag. > > I still have some high level questions not yet got answered. Do you want > to answer them? > > https://lore.kernel.org/all/ad59TxAHNwFWH7Cc@x1.local/ Sorry, reply to this got lost in my TODO list. > In summary, it's about: > > - Whether we have explored other approaches on page hotness tracking So, for read/write tracking we have clear_refs=1, page_idle and DAMON. Did I miss something? clear_refs is process-wide hammer. And you can miss a hot page if it races with LRU rotation. page_idle needs rmap. It will not scale. DAMON is built around sampling. It is good for working set estimation, but I don't think it is directly useful for eviction decision. It can miss hot pages. LRU rotation will also loose info. None of them gives comparable capabilities. We also need a mechanism to atomically evict pages. > - Whether read protection is required for an userspace swap system > (e.g. did you get time to have a look at umap?) I looked at it briefly, so I can miss details. IIUC, in absence of read tracking it doesn't collect hotness information at all. The eviction is based on fault-in time: the oldest faulted-in page gets evicted first. I guess it is fine if you don't care much about refault cost. Like, if your workload fits into memory completely and refaults are rare. That's not my case. -- Kiryl Shutsemau / Kirill A. Shutemov