[RFC v4 PATCH 5/9] mm/hugetlb: vma_has_reserves() needs to handle fallocate hole punch

[Mike Kravetz]

In vma_has_reserves(), the current assumption is that reserves are
always present for shared mappings.  However, will not be the case
with fallocate hole punch.  When punching a hole, the present page
will be deleted as well as the region/reserve map entry (and hence
any reservation).  vma_has_reserves is passed "chg" which indicates
whether or not a region/reserve map is present.  Use this to determine
if reserves are actually present or were removed via hole punch.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/hugetlb.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6881097..ecbaffe 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -692,9 +692,19 @@ static int vma_has_reserves(struct vm_area_struct *vma, long chg)
 			return 0;
 	}
 
-	/* Shared mappings always use reserves */
-	if (vma->vm_flags & VM_MAYSHARE)
-		return 1;
+	if (vma->vm_flags & VM_MAYSHARE) {
+		/*
+		 * We know VM_NORESERVE is not set.  Therefore, there SHOULD
+		 * be a region map for all pages.  The only situation where
+		 * there is no region map is if a hole was punched via
+		 * fallocate.  In this case, there really are no reverves to
+		 * use.  This situation is indicated if chg != 0.
+		 */
+		if (chg)
+			return 0;
+		else
+			return 1;
+	}
 
 	/*
 	 * Only the process that called mmap() has reserves for
-- 
2.1.0

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