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[108.51.163.112]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-50e39495192sm99043001cf.27.2026.04.20.19.21.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Apr 2026 19:21:20 -0700 (PDT) Date: Mon, 20 Apr 2026 22:21:18 -0400 From: Gregory Price To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Andrew Morton , David Hildenbrand , Vlastimil Babka , Brendan Jackman , Michal Hocko , Suren Baghdasaryan , Jason Wang , Andrea Arcangeli , linux-mm@kvack.org, virtualization@lists.linux.dev Subject: Re: [PATCH RFC v2 00/18] mm/virtio: skip redundant zeroing of host-zeroed reported pages Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: jayfjxbs8954q4uotfn4pzsibcr8yegf X-Rspamd-Queue-Id: D065F100014 X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1776738081-731868 X-HE-Meta: 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 clydr7wv j2dgSn55/ZsMTtaARKN9h4PHt+e5cow9+Fwe5l2YyPRc4eldlEcAO84WU2hgW2f+MKBRl+Qr0XQR2zXGi+8d4QYnLUxFJNFEbHWwOiqQrEfvKVb2PinN0RTViZxYCmqz9h/1bhs7e9ULbxSfDg+krk7FGyy4koP9F1Y+sV7MQJZZ/DfEQYvoVdwjvhMAwHy1VWT95oKL9pa6GK+rE9XE0RNrx2XJSZNrZY5Xh8UsdCigMLA+bBD5t26KfdExhEjdaCMq/cPacnq/Hz3FYFBXXF4NHSKf3FhTra5WPsoV8JwdMrzfqwIloVf4h3Pb8ItHANoOStEJnE4mS51w= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Apr 20, 2026 at 08:51:13AM -0400, Michael S. Tsirkin wrote: > > When a guest reports free pages to the hypervisor via virtio-balloon's > free page reporting, the host typically zeros those pages when reclaiming > their backing memory (e.g., via MADV_DONTNEED on anonymous mappings). > When the guest later reallocates those pages, the kernel zeros them > again -- redundantly. > It took me a second to really wrap my head around what you were saying here, but if i'm following correctly: 1) Guest steals a page, reports the free page to the host 2) Host returns that page to the buddy 3) Guest wants the page back -> vmexit, alloc() a) host gets a page from the buddy via fault path b) this memory is "user memory" so host zeroes the page 4) Guest repeats step 3, re-zeoring the page So you're adding a step that does: 1) page_reporting_drain() in guest sets PG_zeroed if host_zeroes_pages=true 2) on allocation, if PG_zeroed is set, don't zero In theory this seems ok. PG_zeroed being a buddy-only flag is nice. In practice there are obvious concerns about an explicit flag that would allow a kernel (in this case the guest) to skip zeroing a page destined for userland mappings - but i'm also paranoid. In concept this seems reasonable, in implementation I have concerns about the pghint_t type being added. Will respond inline in David's reply thread on that though where you already have notes. ~Gregory