From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 4A52739901A; Mon, 13 Apr 2026 08:15:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776068116; cv=none; b=VTe1RmJpcB1eOBZjntVQSLlD/IWT+XiHV+9GcE3V6G/KjAF2mp43V3vH6FQclHmJbfvq9GBIiidNgZ2ZjnEU2Rv4RO7HnR58q4AvjgNUIsDzK9sx3sI8TI7lZe3Wt/2NdmiIGQD9TJ7kDkUIOmRqJmKhzvk7qMiCBlh7kC2VEK4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776068116; c=relaxed/simple; bh=33NTUTIwsDbi+rzZUkVvN5OJH7UDAeSwaboNiGNq9+A=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=pf+0YAsz4cmlAHpLNtjt/+FNjRY5R8t6fyJ8QQy9oqNQqdjIF0nvC8O5P8pcHLgDLv2bb/7dtFIVQ54EqNJ2ksfR9thZ7qB33jBZk3tpLWDLQ9+5W7T0Mw2R3qaLCWXX3SZspkF0kfQzxs9+r+2u5fk+VHMpDh7zsg4FlGKFABI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mbiFSFfT; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mbiFSFfT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1349FC2BCB0; Mon, 13 Apr 2026 08:15:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776068116; bh=33NTUTIwsDbi+rzZUkVvN5OJH7UDAeSwaboNiGNq9+A=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=mbiFSFfTNcEKw5YAOH4DOxMGSmBU3n6+O+zj2mN2C0clNi0DFYW7IwqtZYNSPgWE/ fqZSbHWxiO7a7rDnRcRNeDH/rQjwiPk2mvQemSbQ4p6RSFAF485IKX3d8/UsiSojDS Fqq2ExtLmFLznIr1GYYZw+zchzSAR5WH9da9skaePiJiHkR0Ip1vep7slyLcTFebpU 5BMuJv9XPQR7cgpvGYvQ1uWI6vOOsYZaVi/gdV0b1Kk4wUT3VsgoMgzMXZwIyrbkVt TP5XU6xiYzseMGWJ3cRNQOa6qesQgb9weZ9LuW09e3ZJEz2T0Biq/8OEG+bbf0JCMO wWZy3fyLHZPFQ== Message-ID: <4e1b349b-d0df-4ad5-8050-9a2b3b3831ac@kernel.org> Date: Mon, 13 Apr 2026 10:15:08 +0200 Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 2/9] mm: page_reporting: skip redundant zeroing of host-zeroed reported pages To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Andrew Morton , Vlastimil Babka , Brendan Jackman , Michal Hocko , Suren Baghdasaryan , Jason Wang , Andrea Arcangeli , linux-mm@kvack.org, virtualization@lists.linux.dev, Lorenzo Stoakes , "Liam R. Howlett" , Mike Rapoport , Johannes Weiner , Zi Yan References: <2155527a-e077-4b71-80ee-d735f9984f60@kernel.org> <20260413040143-mutt-send-email-mst@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260413040143-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/13/26 10:10, Michael S. Tsirkin wrote: > On Mon, Apr 13, 2026 at 10:00:58AM +0200, David Hildenbrand (Arm) wrote: >> On 4/13/26 00:50, Michael S. Tsirkin wrote: >>> When a guest reports free pages to the hypervisor via the page reporting >>> framework (used by virtio-balloon and hv_balloon), the host typically >>> zeros those pages when reclaiming their backing memory. However, when >>> those pages are later allocated in the guest, post_alloc_hook() >>> unconditionally zeros them again if __GFP_ZERO is set. This >>> double-zeroing is wasteful, especially for large pages. >>> >>> Avoid redundant zeroing by propagating the "host already zeroed this" >>> information through the allocation path: >>> >>> 1. Add a host_zeroes_pages flag to page_reporting_dev_info, allowing >>> drivers to declare that their host zeros reported pages on reclaim. >>> A static key (page_reporting_host_zeroes) gates the fast path. >>> >>> 2. In page_del_and_expand(), when the page was reported and the >>> static key is enabled, stash a sentinel value (MAGIC_PAGE_ZEROED) >>> in page->private. >>> >>> 3. In post_alloc_hook(), check page->private for the sentinel. If >>> present and zeroing was requested (but not tag zeroing), skip >>> kernel_init_pages(). >>> >>> In particular, __GFP_ZERO is used by the x86 arch override of >>> vma_alloc_zeroed_movable_folio. >>> >>> No driver sets host_zeroes_pages yet; a follow-up patch to >>> virtio_balloon is needed to opt in. >>> >>> Signed-off-by: Michael S. Tsirkin >>> Assisted-by: Claude:claude-opus-4-6 >>> --- >>> include/linux/mm.h | 6 ++++++ >>> include/linux/page_reporting.h | 3 +++ >>> mm/page_alloc.c | 21 +++++++++++++++++++++ >>> mm/page_reporting.c | 9 +++++++++ >>> mm/page_reporting.h | 2 ++ >>> 5 files changed, 41 insertions(+) >>> >>> diff --git a/include/linux/mm.h b/include/linux/mm.h >>> index 5be3d8a8f806..59fc77c4c90e 100644 >>> --- a/include/linux/mm.h >>> +++ b/include/linux/mm.h >>> @@ -4814,6 +4814,12 @@ static inline bool user_alloc_needs_zeroing(void) >>> &init_on_alloc); >>> } >>> >>> +/* >>> + * Sentinel stored in page->private to indicate the page was pre-zeroed >>> + * by the hypervisor (via free page reporting). >>> + */ >>> +#define MAGIC_PAGE_ZEROED 0x5A45524FU /* ZERO */ >> >> Why are we not using another page flag that is yet unused for buddy pages? > > Because we need to report the status *after* it left buddy. > And all flags are in use at that point. I'll comment on that on the other patch, where __GFP_PREZEROED, which I really hate, is added. > > >> Using page->private for that, and exposing it to buddy users with the >> __GFP_PREZEROED flag (I hope we can avoid that) does not sound >> particularly elegant. > > But propagating this all over mm does not sound too palatable, right? > There's precedent with MAGIC_HWPOISON already. > Better ideas? Thanks! I'll comment on the __GFP_PREZEROED patch. > >> Also, if we're going to remember that some pages in the buddy are >> pre-zeroed, it should better not be free-page-reporting specific. >> I'd assume ordinary inflating+deflating of the balloon would also end up >> with pre-zeroed pages. We'd just need a (mm/balloon.c -specific) >> interface to tell the buddy that the pages are zeroed. >> > > Indeed, it's also easily possible - it's a separate optimization, though. > Another simple enhancement is including hugetlbfs freelists in page > reporting. > Doesn't need to block this patchset though, right? Not blocking, but I don't want something that is too coupled to free-page reporting optimizations in the buddy. The comment above MAGIC_PAGE_ZEROED triggered my reaction. -- Cheers, David