[RFC PATCH 1/3] mm: enable MADV_DONTNEED for hugetlb mappings

[Mike Kravetz <mike.kravetz@oracle.com>]

MADV_DONTNEED is currently disabled for hugetlb mappings.  This
certainly makes sense in shared file mappings as the pagecache maintains
a reference to the page and it will never be freed.  However, it could
be useful to unmap and free pages in private mappings.

The only thing preventing MADV_DONTNEED (and MADV_FREE) from working on
hugetlb mappings is a check in can_madv_lru_vma().  To allow support for
hugetlb mappings create and use a new routine madvise_dontneed_valid_vma()
that will allow hugetlb mappings.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 mm/madvise.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 8c927202bbe6..fc8992f4ae40 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -766,6 +766,11 @@ static long madvise_dontneed_single_vma(struct vm_area_struct *vma,
 	return 0;
 }
 
+static bool madvise_dontneed_valid_vma(struct vm_area_struct *vma)
+{
+	return (vma->vm_flags & VM_HUGETLB) || can_madv_lru_vma(vma);
+}
+
 static long madvise_dontneed_free(struct vm_area_struct *vma,
 				  struct vm_area_struct **prev,
 				  unsigned long start, unsigned long end,
@@ -774,7 +779,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
 	struct mm_struct *mm = vma->vm_mm;
 
 	*prev = vma;
-	if (!can_madv_lru_vma(vma))
+	if (!madvise_dontneed_valid_vma(vma))
 		return -EINVAL;
 
 	if (!userfaultfd_remove(vma, start, end)) {
@@ -796,7 +801,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
 			 */
 			return -ENOMEM;
 		}
-		if (!can_madv_lru_vma(vma))
+		if (!madvise_dontneed_valid_vma(vma))
 			return -EINVAL;
 		if (end > vma->vm_end) {
 			/*
-- 
2.34.1