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    SubjectRe: [PATCH v7 09/24] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM
    On Wed, 20 Nov 2019 23:13:39 -0800
    John Hubbard <jhubbard@nvidia.com> wrote:

    > As it says in the updated comment in gup.c: current FOLL_LONGTERM
    > behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the
    > FS DAX check requirement on vmas.
    >
    > However, the corresponding restriction in get_user_pages_remote() was
    > slightly stricter than is actually required: it forbade all
    > FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers
    > that do not set the "locked" arg.
    >
    > Update the code and comments accordingly, and update the VFIO caller
    > to take advantage of this, fixing a bug as a result: the VFIO caller
    > is logically a FOLL_LONGTERM user.
    >
    > Also, remove an unnessary pair of calls that were releasing and
    > reacquiring the mmap_sem. There is no need to avoid holding mmap_sem
    > just in order to call page_to_pfn().
    >
    > Also, move the DAX check ("if a VMA is DAX, don't allow long term
    > pinning") from the VFIO call site, all the way into the internals
    > of get_user_pages_remote() and __gup_longterm_locked(). That is:
    > get_user_pages_remote() calls __gup_longterm_locked(), which in turn
    > calls check_dax_vmas(). It's lightly explained in the comments as well.
    >
    > Thanks to Jason Gunthorpe for pointing out a clean way to fix this,
    > and to Dan Williams for helping clarify the DAX refactoring.
    >
    > Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
    > Reviewed-by: Ira Weiny <ira.weiny@intel.com>
    > Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
    > Cc: Dan Williams <dan.j.williams@intel.com>
    > Cc: Jerome Glisse <jglisse@redhat.com>
    > Signed-off-by: John Hubbard <jhubbard@nvidia.com>
    > ---
    > drivers/vfio/vfio_iommu_type1.c | 30 +++++-------------------------
    > mm/gup.c | 27 ++++++++++++++++++++++-----
    > 2 files changed, 27 insertions(+), 30 deletions(-)

    Tested with device assignment and Intel mdev vGPU assignment with QEMU
    userspace:

    Tested-by: Alex Williamson <alex.williamson@redhat.com>
    Acked-by: Alex Williamson <alex.williamson@redhat.com>

    Feel free to include for 19/24 as well. Thanks,

    Alex

    > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
    > index d864277ea16f..c7a111ad9975 100644
    > --- a/drivers/vfio/vfio_iommu_type1.c
    > +++ b/drivers/vfio/vfio_iommu_type1.c
    > @@ -340,7 +340,6 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
    > {
    > struct page *page[1];
    > struct vm_area_struct *vma;
    > - struct vm_area_struct *vmas[1];
    > unsigned int flags = 0;
    > int ret;
    >
    > @@ -348,33 +347,14 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
    > flags |= FOLL_WRITE;
    >
    > down_read(&mm->mmap_sem);
    > - if (mm == current->mm) {
    > - ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page,
    > - vmas);
    > - } else {
    > - ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
    > - vmas, NULL);
    > - /*
    > - * The lifetime of a vaddr_get_pfn() page pin is
    > - * userspace-controlled. In the fs-dax case this could
    > - * lead to indefinite stalls in filesystem operations.
    > - * Disallow attempts to pin fs-dax pages via this
    > - * interface.
    > - */
    > - if (ret > 0 && vma_is_fsdax(vmas[0])) {
    > - ret = -EOPNOTSUPP;
    > - put_page(page[0]);
    > - }
    > - }
    > - up_read(&mm->mmap_sem);
    > -
    > + ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
    > + page, NULL, NULL);
    > if (ret == 1) {
    > *pfn = page_to_pfn(page[0]);
    > - return 0;
    > + ret = 0;
    > + goto done;
    > }
    >
    > - down_read(&mm->mmap_sem);
    > -
    > vaddr = untagged_addr(vaddr);
    >
    > vma = find_vma_intersection(mm, vaddr, vaddr + 1);
    > @@ -384,7 +364,7 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
    > if (is_invalid_reserved_pfn(*pfn))
    > ret = 0;
    > }
    > -
    > +done:
    > up_read(&mm->mmap_sem);
    > return ret;
    > }
    > diff --git a/mm/gup.c b/mm/gup.c
    > index 14fcdc502166..cce2c9676853 100644
    > --- a/mm/gup.c
    > +++ b/mm/gup.c
    > @@ -29,6 +29,13 @@ struct follow_page_context {
    > unsigned int page_mask;
    > };
    >
    > +static __always_inline long __gup_longterm_locked(struct task_struct *tsk,
    > + struct mm_struct *mm,
    > + unsigned long start,
    > + unsigned long nr_pages,
    > + struct page **pages,
    > + struct vm_area_struct **vmas,
    > + unsigned int flags);
    > /*
    > * Return the compound head page with ref appropriately incremented,
    > * or NULL if that failed.
    > @@ -1167,13 +1174,23 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
    > struct vm_area_struct **vmas, int *locked)
    > {
    > /*
    > - * FIXME: Current FOLL_LONGTERM behavior is incompatible with
    > + * Parts of FOLL_LONGTERM behavior are incompatible with
    > * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
    > - * vmas. As there are no users of this flag in this call we simply
    > - * disallow this option for now.
    > + * vmas. However, this only comes up if locked is set, and there are
    > + * callers that do request FOLL_LONGTERM, but do not set locked. So,
    > + * allow what we can.
    > */
    > - if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
    > - return -EINVAL;
    > + if (gup_flags & FOLL_LONGTERM) {
    > + if (WARN_ON_ONCE(locked))
    > + return -EINVAL;
    > + /*
    > + * This will check the vmas (even if our vmas arg is NULL)
    > + * and return -ENOTSUPP if DAX isn't allowed in this case:
    > + */
    > + return __gup_longterm_locked(tsk, mm, start, nr_pages, pages,
    > + vmas, gup_flags | FOLL_TOUCH |
    > + FOLL_REMOTE);
    > + }
    >
    > return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
    > locked,

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