On Fri, May 05, 2023 at 10:21:21PM +0200, David Hildenbrand wrote: > 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! Thanks! > > 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]. > Nice! I was using the cow selftests as just looking for something that touches FOLL_LONGTERM with PUP_fast, I hacked it so it always wrote just to test patches but clearly we need something more thorough. > > 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? Yeah I noticed this discrepancy when going through initial attempts to refactor in the vmas patch series, I wonder how important it is to differentiate? I have a feeling it probably doesn't matter too much but obviously need input from Jens and Pavel. > > > (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. The entire intent of this series (for me :)) was to allow io_uring to just drop this code altogether so we can unblock my drop the 'vmas' parameter from GUP series [1]. I always intended to respin that after this settled down, Jens and Pavel seemed onboard with this (and really they shouldn't need to be doing that check, that was always a failing in GUP). I will do a v5 of this soon. [1]: https://lore.kernel.org/all/cover.1681831798.git.lstoakes@xxxxxxxxx/ > > > [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 >
