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Sun, 28 Aug 2022 00:59:35 +0000 Received: from BY5PR12MB4130.namprd12.prod.outlook.com ([fe80::508d:221c:9c9e:e1a5]) by BY5PR12MB4130.namprd12.prod.outlook.com ([fe80::508d:221c:9c9e:e1a5%8]) with mapi id 15.20.5566.021; Sun, 28 Aug 2022 00:59:35 +0000 Message-ID: <033c2e8d-8ba9-3e0b-9dee-7d0973c708fa@nvidia.com> Date: Sat, 27 Aug 2022 17:59:32 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH v2] mm: re-allow pinning of zero pfns (again) Content-Language: en-US To: Alex Williamson , akpm@linux-foundation.org Cc: alex.sierra@amd.com, willy@infradead.org, hch@lst.de, Felix.Kuehling@amd.com, apopple@nvidia.com, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <166015037385.760108.16881097713975517242.stgit@omen> From: John Hubbard In-Reply-To: <166015037385.760108.16881097713975517242.stgit@omen> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SJ0PR03CA0242.namprd03.prod.outlook.com (2603:10b6:a03:3a0::7) To BY5PR12MB4130.namprd12.prod.outlook.com (2603:10b6:a03:20b::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 75e20251-8e62-43c3-464b-08da889092e8 X-MS-TrafficTypeDiagnostic: CY4PR12MB1829:EE_ X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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imf07.hostedemail.com; dkim=pass header.d=Nvidia.com header.s=selector2 header.b=T5I+MEul; arc=pass ("microsoft.com:s=arcselector9901:i=1"); spf=pass (imf07.hostedemail.com: domain of jhubbard@nvidia.com designates 40.107.243.60 as permitted sender) smtp.mailfrom=jhubbard@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com ARC-Seal: i=2; s=arc-20220608; d=hostedemail.com; t=1661648379; a=rsa-sha256; cv=pass; b=qVG2o4PWApROwWwofTn0DAVOAgsWm9LTvtt76oYAEIsk6iMrAyDVt5GNtlM+mHWMgltPUH jxeJEwvddpTwxg2zAoaZXTvtwMVyF3tq/ZefRrzJ3+Po0OPbmn83BN+EXFSAsbS/8xlsEM 7LJEwM2GewO2LQuor3J5KDpn2SJ63bY= ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661648379; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=cfRYng4ouihPTg3N5FABq630ffCumSEItstXVRQQiWg=; b=m9n80pZV/aK5bCxIFgvVUzKde5+iKo2qwE0AGJFEl3DCbWq1wxq/9uh75Jt6nRCiAnp8Zb Ud5vrYLTphjbfCV6psBXU4WRViITBioG87bqqMBdqbIxAf0NLQbQ/ZIourPXc3ZRAkju9U vfFCGjO4ZmpVOkUcsZ00my0Y4uHMWsU= Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=Nvidia.com header.s=selector2 header.b=T5I+MEul; arc=pass ("microsoft.com:s=arcselector9901:i=1"); spf=pass (imf07.hostedemail.com: domain of jhubbard@nvidia.com designates 40.107.243.60 as permitted sender) smtp.mailfrom=jhubbard@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com X-Stat-Signature: 6mjeikz791m59p7zshzhspkayhkgubwh X-Rspamd-Queue-Id: 0A67F4002C X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1661648377-547377 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 8/10/22 09:53, Alex Williamson wrote: > The below referenced commit makes the same error as 1c563432588d ("mm: fix > is_pinnable_page against a cma page"), re-interpreting the logic to exclude > pinning of the zero page, which breaks device assignment with vfio. > > To avoid further subtle mistakes, split the logic into discrete tests. > > Suggested-by: Matthew Wilcox > Suggested-by: Felix Kuehling > Link: https://lore.kernel.org/all/165490039431.944052.12458624139225785964.stgit@omen > Fixes: f25cbb7a95a2 ("mm: add zone device coherent type memory support") > Signed-off-by: Alex Williamson > --- > include/linux/mm.h | 17 ++++++++++++++--- > 1 file changed, 14 insertions(+), 3 deletions(-) Hi Alex, Looks good. I'm suggesting a simpler comment, below, because even though the VFIO folks are thinking about VFIO, here we are deep in the mm layer and there are lots of non-VFIO callers that may pin the zero page. > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 18e01474cf6b..835106a9718f 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1544,9 +1544,20 @@ static inline bool is_longterm_pinnable_page(struct page *page) > if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE) > return false; > #endif > - return !(is_device_coherent_page(page) || > - is_zone_movable_page(page) || > - is_zero_pfn(page_to_pfn(page))); > + /* > + * The zero page might reside in a movable zone, however it may not > + * be migrated and can therefore be pinned. The vfio subsystem pins > + * user mappings including the zero page for IOMMU translation. > + */ Those notes are all about (some of) the callers. But it's a simple answer, really, so how about just this: /* The zero page is always allowed to be pinned. */ ? > + if (is_zero_pfn(page_to_pfn(page))) > + return true; > + > + /* Coherent device memory must always allow eviction. */ > + if (is_device_coherent_page(page)) > + return false; > + > + /* Otherwise, non-movable zone pages can be pinned. */ > + return !is_zone_movable_page(page); > } > #else > static inline bool is_longterm_pinnable_page(struct page *page) > > > Reviewed-by: John Hubbard thanks, -- John Hubbard NVIDIA