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Tue, 14 Apr 2026 13:10:40 -0400 (EDT) Date: Tue, 14 Apr 2026 18:10:35 +0100 From: Kiryl Shutsemau To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Peter Xu , 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: <20260414142354.1465950-1-kas@kernel.org> <55019037-4f1c-4d9c-83ee-3a844d8f3d5e@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <55019037-4f1c-4d9c-83ee-3a844d8f3d5e@kernel.org> X-Rspamd-Queue-Id: BB95218000D X-Stat-Signature: rw8im3bnc77838dyqs6685m49g8ig1es X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1776186645-560252 X-HE-Meta: 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 TuZq/0Qc pah5H+7L7YkgEO7yn22zA0H0WXg1urq40MysnII101dSNuF5BdhBtj6Lfy7uOL0DipQNXRDR2OUNacTtQa+UrlhlSCAxGGOfOlMOKQOWQk9ze+lYAqieLarRemDfGspcluf36WERLEGsvLzeB6v2/JrBOtQzefQeKt06O+EkF66KrtPbZyaihkU7CqGoqUhmlbAR8F+CbYBr35mX1bQ1dKVUhP9KP4N/aHaY0PUazkdEp3Fms93uXLFwSQ3EygZKDjA2fhOzxKxhRs9TfZ4PfBLlf6/cA28lfOVxkgg+q6S8LZPc= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Apr 14, 2026 at 05:37:50PM +0200, David Hildenbrand (Arm) wrote: > On 4/14/26 16:23, Kiryl Shutsemau (Meta) wrote: > > This series adds userfaultfd support for tracking the working set of > > VM guest memory, enabling VMMs to identify cold pages and evict them > > to tiered or remote storage. > > > > == Problem == > > > > VMMs managing guest memory need to: > > 1. Track which pages are actively used (working set detection) > > 2. Safely evict cold pages to slower storage > > 3. Fetch pages back on demand when accessed again > > > > For shmem-backed guest memory, working set tracking partially works > > today: MADV_DONTNEED zaps PTEs while pages stay in page cache, and > > re-access auto-resolves from cache. But safe eviction still requires > > synchronous fault interception to prevent data loss races. > > > > For anonymous guest memory (needed for KSM cross-VM deduplication), > > there is no mechanism at all — clearing a PTE loses the page. > > > > == Solution == > > > > The series introduces a unified userfaultfd interface that works > > across both anonymous and shmem-backed memory: > > > > UFFD_FEATURE_MINOR_ANON: extends MODE_MINOR registration to anonymous > > private memory. Uses the PROT_NONE hinting mechanism (same as NUMA > > balancing) to make pages inaccessible without freeing them. > > I would rather tackle this from the other direction: it's another form > of protection (like WP), not really a "minor" mode. > > Could we add a UFFDIO_REGISTER_MODE_RWP (or however we would call it) > and support it for anon+shmem, avoiding the zapping for shmem completely? I like this idea. It should be functionally equivalent, but your interface idea fits better with the rest. Thanks! Will give it a try. -- Kiryl Shutsemau / Kirill A. Shutemov