From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 160E533F58B; Mon, 8 Jun 2026 11:13:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780917233; cv=none; b=WtFwqVzgabWA2cZjtHn4g6vOyVbu2BImpRXP9ks+/gEfRjvtIkp2SI7Nd4UIfgUENnIOszzHWEebsrRYNKrPOgWOs5sRfz+MhN2hUNNeOzJiJwMNs5v9shgXWjYls3OARdHAX9Gen9jGDll/iX2pzIDJyhp27zch7MvlgfDCQuM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780917233; c=relaxed/simple; bh=KC+uicU/B9ZZgU3DjZNygswy0QSjvcRUVDgEaOeI9gA=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=M4lz3WH/Q9Pbg3B2FZ9rGsng76D87BEhKfMicsU84IXdQeUfOaS6BeOFpuamyp9NhcX9KGXwuR84SnYu3vN2EAJbDo7WdgxE6B0T1YbEJnAR5J2Zew6uMTYZSyJo6oiLMFjfiPsu5aMBqehsHxWKE73j/tvOj4aH57CecZ++BVI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dVO2Xhpt; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="dVO2Xhpt" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8544A1F00893; Mon, 8 Jun 2026 11:13:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780917231; bh=XGLJigV2d51VfcYTfl2W0053EcxmumDAe7qlHgw2QFo=; h=Date:Subject:From:To:Cc:References:In-Reply-To; b=dVO2Xhpt5B9et3HtGZ+E5d1dDwlrohdKp3ui6s8ZawWlFdNVIF820M+BYU1lU2ait w0NEv49grfPSyTsnHbfNTxMPoe0jIAi8DcOTYsU7E1pz4q/MFzX7BDm64USjSZTrB/ f5sMNgZUD514qMoBCoyZ4GHXnNTKDoHU26W1GuAv3Q3TRjHqCGUdc1CadbTSYYHaEL rCfZ76HtL+z/LqyCEwsRsYwHI2jhvjI/LHBh2UThS/oQM2jDp0lbqpVpgU38tkL2B9 6NTz8VUDL8inCaOfCwgC1iXHvtXQVuNQ0FIU8eJgspROwelNNwbqAJFWgcINsPcoVM AAgE0rkSS6xxQ== Message-ID: Date: Mon, 8 Jun 2026 13:13:42 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 00/37] mm/virtio: skip redundant zeroing of host-zeroed pages Content-Language: en-US From: "Vlastimil Babka (SUSE)" To: "Michael S. Tsirkin" , linux-kernel@vger.kernel.org Cc: "David Hildenbrand (Arm)" , Jason Wang , Xuan Zhuo , =?UTF-8?Q?Eugenio_P=C3=A9rez?= , Muchun Song , Oscar Salvador , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Hugh Dickins , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Axel Rasmussen , Yuanchu Xie , Wei Xu , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , virtualization@lists.linux.dev, linux-mm@kvack.org, Andrea Arcangeli References: <8d5d2fa3-a60c-4d04-b3bf-7a8fe89cb1a0@kernel.org> Autocrypt: addr=vbabka@kernel.org; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSNWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBrZXJuZWwub3JnPsLBsAQTAQoAWhYhBKlA1DSZLC6OmRA9UCJPp+fM gqZkBQJqFFy6GxSAAAAAAAQADm1hbnUyLDIuNSsxLjEyLDIsMgIbAwUJGtCBUAULCQgHAwUV CgkICwUWAgMBAAIeBQIXgAAKCRAiT6fnzIKmZJIUEADFx/tREzUImHrEwVHeSvDFmA7tJysI UVrlvrM09E7GIuzphzv7jYmo8n3ANpCczLEVr4G0syYQdTigaZgv3+FQDIIzhKih1IHhu1Ei XHlywNWKnQxxQEUNi5Mwx43wQz5XVw9F1A7gtKBKNtfogO511hAbrzagrYajyQacEJ/+sfhZ 9Da8ltHIXD8pcYaHUfQgEusCgmEd9+KrUwrTbckFKmYq5chuE6yJ4J0EmWknL096jIE6CnzF FRslQ3B1UKDjxVsm1ZHfir5NeWszLkTvGFsddFaWTgh8UycESG6VQzKXjjewXu2pG7YQYRpj QKm1W5X2TkwWkXRBZTmfmbhxIUMh3+zf5wQ463rSmDN/8v81tdqBtAW6rH/kzg1GvkaTHXn0 507yEHFzBksk2viAuIxxr7km8+/KARYLIdGtx30EG8cKzAUZOK6WqxtNCsXUJNrVE8CWrCaD icoNu7Fs1c5hmPHdSTnU48ce67449DdnO4neLSNhRiGlMHJgfJUmgrxu/hcYeOZ3haWmEQ2w uW1Mh01OHi8QZHCEyAbABrPs9GUgccc/4eYXX9hIgxfSkYzn8f+8NuIFPWl/0uTvjgqU29FQ SbzOLxHq9439Ox40G5mS5eZXRGxITYR+6TXvRGI6P/264jvflnr/pDGUttaikU+0W+1uxgKH cmYbEc7ATQRbGTU1AQgAn0H6UrFiWcovkh6EXVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQ La1PQDUi6j00ChlcR66g9/V0sPIcSutacPKfdKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMh FmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCTsTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sf bAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZOrIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq +aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahKtQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4n jQARAQABwsF8BBgBCgAmAhsMFiEEqUDUNJksLo6ZED1QIk+n58yCpmQFAmfIHFQFCRYU6J8A CgkQIk+n58yCpmS2PA//bqN1LfcotmArgElsa+0EGZSQlYgK48pm8WAeTXTngudP9IJ4SuKY HR5RNjHcBeqN+Me0zxRqYzRb8nGanHEkDyf4Im8DQM8d6vbyU+FcPmG4skud4kgS1zMHnlVd SXfSIwKC/hKgdHG8aBV7545Lz9X6Iohea+94wneD0aw/hqF+QWewGZhWJriWAZtvEkzNjQOi 4U9F/trLten/x7bpphDSnDMKJtITbtzATT1Dq7o7VpIUK1nCTQALMuMjKCdi8OdU/+V+R3O4 0PXWvX8qrvqYapVbZ+9KqT74FsuB0Ya9uXwgBF2Q6cRuETZk5vqaqKxzqoQZCO8AOz/58j6O 2RHNy/mZEN+7tJ5Tsq42zVJ4jxsT8b9YplavCMsnBgDeRWhcbYhCyttoL7nYISyWg4kQYZ/P wIV3OuNv2f8iKYsxNsRuClOAF82+gvqOy1/1pprFjy8uo2pkoOrb63aOP3vO5VHnRKgra6dq NcaZ+c6J4H+nEJGi2SkHAUJz5oBzuThvPudLvPA/SK8sKoM01IRxSihev/S/5WLazXB1PGem OCbvzC1IjWJJraxiDJ5IygokapUa2RP7+WBR22skQ3SSl6G107QgWKSyTOGWEaRmV53vxQLV jXuCmzSSasTL60zq5yGrT4/DYQVSNEUiUbG4pYekxJujNeEDkUlky0Y= In-Reply-To: <8d5d2fa3-a60c-4d04-b3bf-7a8fe89cb1a0@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/8/26 13:02, Vlastimil Babka (SUSE) wrote: > On 6/8/26 10:33, Michael S. Tsirkin wrote: >> Further, on architectures with aliasing caches, upstream with init_on_alloc > > It seems those are niche architectures so we can ignore that part for perf > purposes; the other reason why user_alloc_needs_zeroing() would be true is > booting with init_on_alloc. OK I misread how user_alloc_needs_zeroing() works wrt init_on_alloc, as it's negated. But you're changing that anyway to skip that user zeroing, right? " This series eliminates that double-zeroing by moving the zeroing into the post_alloc_hook + propagating the "host already zeroed this page" information through the buddy allocator. " So relying on "everything in buddy is zeroed" would still work I'd think. >> double-zeros user pages: once via kernel_init_pages() in >> post_alloc_hook, and again via clear_user_highpage() at the >> callsite (because user_alloc_needs_zeroing() returns true). >> This series eliminates that double-zeroing by moving the zeroing >> into the post_alloc_hook + propagating the "host >> already zeroed this page" information through the buddy allocator. > > I wonder if this whole thing would be much easier if those that would want > performance would enable only init_on_free and not init_on_alloc at the same > time. If you enable both you're paranoid and just eat the cost I guess. Then > you're maybe also paranoid so much that you wouldn't want to trust the host > zeroing anyway? > > post_alloc_hook() already has logic that if init_on_free is enabled, there's > no init during alloc. So then I think you would only need to communicate > that host zeroed some pages when the guest adds them to the buddy allocator, > and that's it? > > I.e. if we can just assume that everything in the buddy is zeroed, we don't > need all the PG_zero flag and whatnot complexity.