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[2003:cb:c71f:6900:2b25:fc69:599e:3986]) by smtp.gmail.com with ESMTPSA id q18-20020a056000137200b003063176ef09sm3377323wrz.6.2023.05.05.13.21.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 05 May 2023 13:21:24 -0700 (PDT) Message-ID: <6e96358e-bcb5-cc36-18c3-ec5153867b9a@redhat.com> Date: Fri, 5 May 2023 22:21:21 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Content-Language: en-US To: Lorenzo Stoakes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jens Axboe Cc: Jason Gunthorpe , Jens Axboe , Matthew Wilcox , Dennis Dalessandro , Leon Romanovsky , Christian Benvenuti , Nelson Escobar , Bernard Metzler , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Bjorn Topel , Magnus Karlsson , Maciej Fijalkowski , Jonathan Lemon , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org, Oleg Nesterov , Jason Gunthorpe , John Hubbard , Jan Kara , "Kirill A . Shutemov" , Pavel Begunkov , Mika Penttila , Dave Chinner , Theodore Ts'o , Peter Xu , Matthew Rosato , "Paul E . McKenney" , Christian Borntraeger References: From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed mappings by default In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org On 04.05.23 23:27, Lorenzo Stoakes wrote: > Writing to file-backed mappings which require folio dirty tracking using > GUP is a fundamentally broken operation, as kernel write access to GUP > mappings do not adhere to the semantics expected by a file system. > > A GUP caller uses the direct mapping to access the folio, which does not > cause write notify to trigger, nor does it enforce that the caller marks > the folio dirty. > > The problem arises when, after an initial write to the folio, writeback > results in the folio being cleaned and then the caller, via the GUP > interface, writes to the folio again. > > As a result of the use of this secondary, direct, mapping to the folio no > write notify will occur, and if the caller does mark the folio dirty, this > will be done so unexpectedly. > > For example, consider the following scenario:- > > 1. A folio is written to via GUP which write-faults the memory, notifying > the file system and dirtying the folio. > 2. Later, writeback is triggered, resulting in the folio being cleaned and > the PTE being marked read-only. > 3. The GUP caller writes to the folio, as it is mapped read/write via the > direct mapping. > 4. The GUP caller, now done with the page, unpins it and sets it dirty > (though it does not have to). > > This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As > pin_user_pages_fast_only() does not exist, we can rely on a slightly > imperfect whitelisting in the PUP-fast case and fall back to the slow case > should this fail. > > Thanks a lot, this looks pretty good to me! I started writing some selftests (assuming none would be in the works) using iouring and and the gup_tests interface. So far, no real surprises for the general GUP interaction [1]. There are two things I noticed when registering an iouring fixed buffer (that differ now from generic gup_test usage): (1) Registering a fixed buffer targeting an unsupported MAP_SHARED FS file now fails with EFAULT (from pin_user_pages()) instead of EOPNOTSUPP (from io_pin_pages()). The man page for io_uring_register documents: EOPNOTSUPP User buffers point to file-backed memory. ... we'd have to do some kind of errno translation in io_pin_pages(). But the translation is not simple (sometimes we want to forward EOPNOTSUPP). That also applies once we remove that special-casing in io_uring code. ... maybe we can simply update the manpage (stating that older kernels returned EOPNOTSUPP) and start returning EFAULT? (2) Registering a fixed buffer targeting a MAP_PRIVATE FS file fails with EOPNOTSUPP (from io_pin_pages()). As discussed, there is nothing wrong with pinning all-anon pages (resulting from breaking COW). That could be easily be handled (allow any !VM_MAYSHARE), and would automatically be handled once removing the iouring special-casing. [1] # ./pin_longterm # [INFO] detected hugetlb size: 2048 KiB # [INFO] detected hugetlb size: 1048576 KiB TAP version 13 1..50 # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd ok 1 Pinning succeeded as expected # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with tmpfile ok 2 Pinning succeeded as expected # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with local tmpfile ok 3 Pinning failed as expected # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB) ok 4 # SKIP need more free huge pages # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (1048576 kB) ok 5 Pinning succeeded as expected # [RUN] R/W longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd ok 6 Pinning succeeded as expected # [RUN] R/W longterm GUP-fast pin in MAP_SHARED file mapping ... with tmpfile ok 7 Pinning succeeded as expected # [RUN] R/W longterm GUP-fast pin in MAP_SHARED file mapping ... with local tmpfile ok 8 Pinning failed as expected # [RUN] R/W longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB) ok 9 # SKIP need more free huge pages # [RUN] R/W longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd hugetlb (1048576 kB) ok 10 Pinning succeeded as expected # [RUN] R/O longterm GUP pin in MAP_SHARED file mapping ... with memfd ok 11 Pinning succeeded as expected # [RUN] R/O longterm GUP pin in MAP_SHARED file mapping ... with tmpfile ok 12 Pinning succeeded as expected # [RUN] R/O longterm GUP pin in MAP_SHARED file mapping ... with local tmpfile ok 13 Pinning succeeded as expected # [RUN] R/O longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB) ok 14 # SKIP need more free huge pages # [RUN] R/O longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (1048576 kB) ok 15 Pinning succeeded as expected # [RUN] R/O longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd ok 16 Pinning succeeded as expected # [RUN] R/O longterm GUP-fast pin in MAP_SHARED file mapping ... with tmpfile ok 17 Pinning succeeded as expected # [RUN] R/O longterm GUP-fast pin in MAP_SHARED file mapping ... with local tmpfile ok 18 Pinning succeeded as expected # [RUN] R/O longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB) ok 19 # SKIP need more free huge pages # [RUN] R/O longterm GUP-fast pin in MAP_SHARED file mapping ... with memfd hugetlb (1048576 kB) ok 20 Pinning succeeded as expected # [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with memfd ok 21 Pinning succeeded as expected # [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with tmpfile ok 22 Pinning succeeded as expected # [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with local tmpfile ok 23 Pinning succeeded as expected # [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with memfd hugetlb (2048 kB) ok 24 # SKIP need more free huge pages # [RUN] R/W longterm GUP pin in MAP_PRIVATE file mapping ... with memfd hugetlb (1048576 kB) ok 25 Pinning succeeded as expected # [RUN] R/W longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd ok 26 Pinning succeeded as expected # [RUN] R/W longterm GUP-fast pin in MAP_PRIVATE file mapping ... with tmpfile ok 27 Pinning succeeded as expected # [RUN] R/W longterm GUP-fast pin in MAP_PRIVATE file mapping ... with local tmpfile ok 28 Pinning succeeded as expected # [RUN] R/W longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd hugetlb (2048 kB) ok 29 # SKIP need more free huge pages # [RUN] R/W longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd hugetlb (1048576 kB) ok 30 Pinning succeeded as expected # [RUN] R/O longterm GUP pin in MAP_PRIVATE file mapping ... with memfd ok 31 Pinning succeeded as expected # [RUN] R/O longterm GUP pin in MAP_PRIVATE file mapping ... with tmpfile ok 32 Pinning succeeded as expected # [RUN] R/O longterm GUP pin in MAP_PRIVATE file mapping ... with local tmpfile ok 33 Pinning succeeded as expected # [RUN] R/O longterm GUP pin in MAP_PRIVATE file mapping ... with memfd hugetlb (2048 kB) ok 34 # SKIP need more free huge pages # [RUN] R/O longterm GUP pin in MAP_PRIVATE file mapping ... with memfd hugetlb (1048576 kB) ok 35 Pinning succeeded as expected # [RUN] R/O longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd ok 36 Pinning succeeded as expected # [RUN] R/O longterm GUP-fast pin in MAP_PRIVATE file mapping ... with tmpfile ok 37 Pinning succeeded as expected # [RUN] R/O longterm GUP-fast pin in MAP_PRIVATE file mapping ... with local tmpfile ok 38 Pinning succeeded as expected # [RUN] R/O longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd hugetlb (2048 kB) ok 39 # SKIP need more free huge pages # [RUN] R/O longterm GUP-fast pin in MAP_PRIVATE file mapping ... with memfd hugetlb (1048576 kB) ok 40 Pinning succeeded as expected # [RUN] iouring fixed buffer with MAP_SHARED file mapping ... with memfd ok 41 Pinning succeeded as expected # [RUN] iouring fixed buffer with MAP_SHARED file mapping ... with tmpfile ok 42 Pinning succeeded as expected # [RUN] iouring fixed buffer with MAP_SHARED file mapping ... with local tmpfile ok 43 Pinning failed as expected # [RUN] iouring fixed buffer with MAP_SHARED file mapping ... with memfd hugetlb (2048 kB) ok 44 # SKIP need more free huge pages # [RUN] iouring fixed buffer with MAP_SHARED file mapping ... with memfd hugetlb (1048576 kB) ok 45 Pinning succeeded as expected # [RUN] iouring fixed buffer with MAP_PRIVATE file mapping ... with memfd ok 46 Pinning succeeded as expected # [RUN] iouring fixed buffer with MAP_PRIVATE file mapping ... with tmpfile ok 47 Pinning succeeded as expected # [RUN] iouring fixed buffer with MAP_PRIVATE file mapping ... with local tmpfile not ok 48 Pinning failed as expected # [RUN] iouring fixed buffer with MAP_PRIVATE file mapping ... with memfd hugetlb (2048 kB) ok 49 # SKIP need more free huge pages # [RUN] iouring fixed buffer with MAP_PRIVATE file mapping ... with memfd hugetlb (1048576 kB) ok 50 Pinning succeeded as expected Bail out! 1 out of 50 tests failed # Totals: pass:39 fail:1 xfail:0 xpass:0 skip:10 error:0 -- Thanks, David / dhildenb