On 11/21/19 1:35 PM, Alex Williamson wrote:

On Wed, 20 Nov 2019 23:13:39 -0800
John Hubbard [off-list ref] wrote:

quoted

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 <redacted>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Dan Williams <redacted>
Cc: Jerome Glisse <redacted>
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 <redacted>
Acked-by: Alex Williamson <redacted>

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

Alex

Great! Thanks for the testing and ack on those. I'm about to repackage
(and split up as CH requested) for 5.5, and will keep you on CC, of course.

thanks,
-- 
John Hubbard
NVIDIA