On Thu, Nov 14, 2019 at 09:53:25PM -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 <redacted>
Cc: Jerome Glisse <redacted>
Cc: Ira Weiny <ira.weiny@intel.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(-)

Looks OK now  

Reviewed-by: Jason Gunthorpe <redacted>