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From:   Lorenzo Stoakes <lstoakes@gmail.com>
To:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>
Subject: [PATCH v2 0/7] remove the vmas parameter from GUP APIs
Date:   Sat, 15 Apr 2023 10:06:06 +0100
Message-Id: <cover.1681547405.git.lstoakes@gmail.com>
MIME-Version: 1.0
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Precedence: bulk

(pin_/get)_user_pages[_remote]() each provide an optional output parameter
for an array of VMA objects associated with each page in the input range.

These provide the means for VMAs to be returned, as long as mm->mmap_lock
is never released during the GUP operation (i.e. the internal flag
FOLL_UNLOCKABLE is not specified).

In addition, these VMAs have also to only be accessed under the mmap_lock,
and become invalidated the moment it is released.

The vast majority of invocations do not use this functionality and of those
that do, all but one retrieve a single VMA to perform checks upon.

It is not egregious in the single VMA cases to simply replace the operation
with a vma_lookup(). In these cases we duplicate the (fast) lookup on a
slow path already under the mmap_lock.

The special case is io_uring, where io_pin_pages() specifically needs to
assert that all the VMAs possess the same vm->vm_file (possibly NULL) and
they are either anonymous or hugetlb pages.

To continue to provide this functionality, we introduce the FOLL_SAME_PAGE
flag which asserts that the vma->vm_file remains the same throughout,
erroring out if this is not the case.

We can then replace the io_uring case by passing FOLL_SAME_FILE and looking
up the first VMA manually and performing the required checks on this
alone. The combination of the two amount to the same checks being
performed (and avoids an allocation).

Eliminating this parameter eliminates an entire class of errors - the vmas
array used to become a set of dangling pointers if access after release of
mmap_lock was attempted, this is simply no longer possible.

In addition the API is simplified and now clearly expresses what it is for
- applying the specified GUP flags and (if pinning) returning pinned pages.

This change additionally opens the door to further potential improvements
in GUP and the possible marrying of disparate code paths.

I have run the gup_test and a simple io_uring program which exercises the
use of FOLL_SAME_PAGE with no issues.

Thanks to Matthew Wilcox for suggesting this refactoring!

v2:
- Only lookup the VMA if the pin succeeded (other than __access_remote_vm()
  which has different semantics)
- Be pedantically careful about ensuring that under no circumstances can we
  fail to unpin a page

v1:
https://lore.kernel.org/linux-mm/cover.1681508038.git.lstoakes@gmail.com/

Lorenzo Stoakes (7):
  mm/gup: remove unused vmas parameter from get_user_pages()
  mm/gup: remove unused vmas parameter from pin_user_pages_remote()
  mm/gup: remove vmas parameter from get_user_pages_remote()
  mm/gup: introduce the FOLL_SAME_FILE GUP flag
  io_uring: rsrc: use FOLL_SAME_FILE on pin_user_pages()
  mm/gup: remove vmas parameter from pin_user_pages()
  mm/gup: remove vmas array from internal GUP functions

 arch/arm64/kernel/mte.c                    |   7 +-
 arch/powerpc/mm/book3s64/iommu_api.c       |   2 +-
 arch/s390/kvm/interrupt.c                  |   2 +-
 arch/x86/kernel/cpu/sgx/ioctl.c            |   2 +-
 drivers/gpu/drm/radeon/radeon_ttm.c        |   2 +-
 drivers/infiniband/hw/qib/qib_user_pages.c |   2 +-
 drivers/infiniband/hw/usnic/usnic_uiom.c   |   2 +-
 drivers/infiniband/sw/siw/siw_mem.c        |   2 +-
 drivers/iommu/iommufd/pages.c              |   4 +-
 drivers/media/v4l2-core/videobuf-dma-sg.c  |   2 +-
 drivers/misc/sgi-gru/grufault.c            |   2 +-
 drivers/vdpa/vdpa_user/vduse_dev.c         |   2 +-
 drivers/vfio/vfio_iommu_type1.c            |   2 +-
 drivers/vhost/vdpa.c                       |   2 +-
 fs/exec.c                                  |   2 +-
 include/linux/hugetlb.h                    |  10 +-
 include/linux/mm.h                         |  10 +-
 include/linux/mm_types.h                   |   2 +
 io_uring/rsrc.c                            |  39 +++----
 kernel/events/uprobes.c                    |  12 +-
 mm/gup.c                                   | 121 ++++++++-------------
 mm/gup_test.c                              |  14 +--
 mm/hugetlb.c                               |  24 ++--
 mm/memory.c                                |   9 +-
 mm/process_vm_access.c                     |   2 +-
 mm/rmap.c                                  |   2 +-
 net/xdp/xdp_umem.c                         |   2 +-
 security/tomoyo/domain.c                   |   2 +-
 virt/kvm/async_pf.c                        |   3 +-
 virt/kvm/kvm_main.c                        |   2 +-
 30 files changed, 127 insertions(+), 164 deletions(-)

--
2.40.0