Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

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.

Thanks to Jason Gunthorpe for pointing out a clean way to fix this.

Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jerome Glisse <redacted>
Cc: Ira Weiny <redacted>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 drivers/vfio/vfio_iommu_type1.c | 30 +++++++++++++-----------------
 mm/gup.c                        | 13 ++++++++-----
 2 files changed, 21 insertions(+), 22 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index d864277ea16f..017689b7c32b 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c

@@ -348,24 +348,20 @@ 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]);
-               }
+       ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
+                                   page, vmas, NULL);