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 to loosen the restriction, allowing
FOLL_LONGTERM in some cases.

Also, copy 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(). This check will then be removed from the VFIO
call site in a subsequent patch.

Thanks to Jason Gunthorpe for pointing out a clean way to fix this,
and to Dan Williams for helping clarify the DAX refactoring.

Tested-by: Alex Williamson <alex.william...@redhat.com>
Acked-by: Alex Williamson <alex.william...@redhat.com>
Reviewed-by: Jason Gunthorpe <j...@mellanox.com>
Reviewed-by: Ira Weiny <ira.we...@intel.com>
Suggested-by: Jason Gunthorpe <j...@ziepe.ca>
Cc: Dan Williams <dan.j.willi...@intel.com>
Cc: Jerome Glisse <jgli...@redhat.com>
Signed-off-by: John Hubbard <jhubb...@nvidia.com>
---
 mm/gup.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 3ecce297a47f..c0c56888e7cc 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.
@@ -1179,13 +1186,23 @@ 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,
-- 
2.24.0

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