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SubjectRe: [PATCH v4 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM
On Wed, Nov 13, 2019 at 09:02:02AM -0400, Jason Gunthorpe wrote:
> On Tue, Nov 12, 2019 at 08:26:55PM -0800, John Hubbard 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.
> >
> > Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > Cc: Jerome Glisse <jglisse@redhat.com>
> > Cc: Ira Weiny <ira.weiny@intel.com>
> > Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> > drivers/vfio/vfio_iommu_type1.c | 25 ++-----------------------
> > mm/gup.c | 27 ++++++++++++++++++++++-----
> > 2 files changed, 24 insertions(+), 28 deletions(-)
> >
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index d864277ea16f..7301b710c9a4 100644
> > +++ 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 +347,13 @@ 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;
>
> Mind the return with the lock held this needs some goto unwind

Ah yea... retract my reviewed by... :-(

Ira

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