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[2003:cb:c709:600:4306:6a2b:57d5:2da1]) by smtp.gmail.com with ESMTPSA id b5-20020a5d45c5000000b002fda1b12a0bsm2397513wrs.2.2023.05.25.09.47.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 25 May 2023 09:47:53 -0700 (PDT) Message-ID: <97970ae6-e912-912a-1755-8d7bbb1131d0@redhat.com> Date: Thu, 25 May 2023 18:47:52 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [RFC PATCH 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages() To: David Howells , Christoph Hellwig Cc: Jens Axboe , Al Viro , Matthew Wilcox , Jan Kara , Jeff Layton , Jason Gunthorpe , Logan Gunthorpe , Hillf Danton , Christian Brauner , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton References: <20230525155102.87353-1-dhowells@redhat.com> <20230525155102.87353-2-dhowells@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20230525155102.87353-2-dhowells@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: B351140002 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: je1fsuqamtgxf11ndpfse8r7i49u5xb7 X-HE-Tag: 1685033278-224472 X-HE-Meta: U2FsdGVkX1+ErcvYoSZ7Ph1UE6o/ZqhAh53YdCpH7Xs/KJsL7lqeXX8TBGk+Jf7TbhZuIaNl7f10bLtnobHTFSDzC28Y5r/71mp2MYG8r93whjC4VSPvmLVDryL4RAw5+0kRLRNstWhTk+rRSZXAqYstbjOxvIGvBmOZrEA3Z52f5Vf+bQ96mxyoTqlyE9bl6++M2wwRGbSlt3FLBH4c7vRxVdqShk9lBHXf8AqAHCYlwMDlwF8+1DHjOKf1rcVbVWMjMfWxWxjgibGIv/kI+trrR1JaAjNhHsnD2uoYuW//QFC04D/OTS5aip7T28jAAdEjVsrvt51XKTUdDlJyArFGToAe9fqQ3srKEgSkyxX9p4+8tb/Umru9KB619F4ghAYFgi9LKHjeMRioCGv2VRVrB84V86d8pWIXRIyIxneZ9G6n9Ba0MqKcWIFJ4djPmC0r5uqznpg4BOZPMjSNCKWtOS/uNmQf5OMB8VS/tNqbVG4e5la0Lbku85ay0FH9b4heWxw7IV5laxUW3On7kNSkgGjcWPg3lSxyzf9P3mRRLuUQsCxQbPR8Pc4HFk55svJC+qcL+cze7xQnUciiN/frteSFsbHlJ+Ynd5sU/kBgMmt71ZoJHde3+XnZ3uU+eZv1uboVphW3fUGBHmjpK85/iR8iqsKROsvl54p/ymb/htoR9J30B0/So7Oqxfm3G9thvx5dB1gKR8VmkJllhor+LF0Gi70WJMttXXq8AZuNUroOnfcwBEYsoPUIj036r5gfdVLIf7lL5rbzl4gNT5tsHOdAuhAsjxMEhHmvdY6DTqSO2Xygzedhxpmg5Fopfk4OPF7mhY5TUnSHHzB63r80BiUV5tUWwGz+XhlLyR+RbKTXF1g/MY41y/wWKv0b1b3L7G6Id26XoA/OS1EFhqtKKi9DCq402xnZxV0eYQn69YTqLnP64kFvbObqQ7zZ390OFezl/hDHjClaWrT 32wWV41/ 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 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 25.05.23 17:51, David Howells wrote: > Make pin_user_pages*() leave the ZERO_PAGE unpinned if it extracts a > pointer to it from the page tables and make unpin_user_page*() > correspondingly ignore the ZERO_PAGE when unpinning. We don't want to risk > overrunning the zero page's refcount as we're only allowed ~2 million pins > on it - something that userspace can conceivably trigger. > As Linus raised, the ZERO_PAGE(0) checks should probably be is_zero_pfn(page_to_pfn(page)). > Signed-off-by: David Howells > cc: Christoph Hellwig > cc: David Hildenbrand > cc: Andrew Morton > cc: Jens Axboe > cc: Al Viro > cc: Matthew Wilcox > cc: Jan Kara > cc: Jeff Layton > cc: Jason Gunthorpe > cc: Logan Gunthorpe > cc: Hillf Danton > cc: Christian Brauner > cc: Linus Torvalds > cc: linux-fsdevel@vger.kernel.org > cc: linux-block@vger.kernel.org > cc: linux-kernel@vger.kernel.org > cc: linux-mm@kvack.org > --- > mm/gup.c | 25 ++++++++++++++++++++++++- > 1 file changed, 24 insertions(+), 1 deletion(-) > > diff --git a/mm/gup.c b/mm/gup.c > index bbe416236593..d2662aa8cf01 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -51,7 +51,8 @@ static inline void sanity_check_pinned_pages(struct page **pages, > struct page *page = *pages; > struct folio *folio = page_folio(page); > > - if (!folio_test_anon(folio)) > + if (page == ZERO_PAGE(0) || > + !folio_test_anon(folio)) > continue; > if (!folio_test_large(folio) || folio_test_hugetlb(folio)) > VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page); > @@ -131,6 +132,13 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags) > else if (flags & FOLL_PIN) { > struct folio *folio; > > + /* > + * Don't take a pin on the zero page - it's not going anywhere > + * and it is used in a *lot* of places. > + */ > + if (page == ZERO_PAGE(0)) > + return page_folio(ZERO_PAGE(0)); With the fixed check, this should be return page_folio(page); I guess. -- Thanks, David / dhildenb