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
