[PATCH 3/4] mm/hmm: allow hmm_range_fault() of mmap(PROT_NONE)

[Ralph Campbell]

Allow hmm_range_fault() to return success (0) when the range has no access
(!(vma->vm_flags & VM_READ)). The range->pfns[] array will be filled with
range->values[HMM_PFN_NONE] in this case.
This allows the caller to get a snapshot of a range without having to
lookup the vma before calling hmm_range_fault().
If the call to hmm_range_fault() is not a snapshot, the caller can still
check that pfns have the desired access permissions.

Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
---
 mm/hmm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 7217912bef13..16c834e5d1c0 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -795,7 +795,9 @@ static int hmm_vma_walk_test(unsigned long start,
 	 */
 	if (!(vma->vm_flags & VM_READ)) {
 		(void) hmm_pfns_fill(start, end, range, HMM_PFN_NONE);
-		return -EPERM;
+
+		/* Skip this vma and continue processing the next vma. */
+		return 1;
 	}
 
 	return 0;
-- 
2.20.1