[PATCH 34/46] hugetlb: userfaultfd: when using MADV_SPLIT, round addresses to PAGE_SIZE

[James Houghton]

MADV_SPLIT enables HugeTLB HGM which allows for UFFDIO_CONTINUE in
PAGE_SIZE chunks. If a huge-page-aligned address were to be provided,
userspace would be completely unable to take advantage of HGM. That
would then require userspace to know to provide
UFFD_FEATURE_EXACT_ADDRESS.

This patch would make it harder to make a mistake. Instead of requiring
userspace to provide UFFD_FEATURE_EXACT_ADDRESS, always provide a usable
address.

Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
---
 mm/hugetlb.c | 31 +++++++++++++++----------------
 1 file changed, 15 insertions(+), 16 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5af6db52f34e..5b6215e03fe1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5936,28 +5936,27 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
 						  unsigned long addr,
 						  unsigned long reason)
 {
+	u32 hash;
+	struct vm_fault vmf;
+
 	/*
 	 * Don't use the hpage-aligned address if the user has explicitly
 	 * enabled HGM.
 	 */
 	if (hugetlb_hgm_advised(vma) && reason == VM_UFFD_MINOR)
-		haddr = address & PAGE_MASK;
-
-	u32 hash;
-	struct vm_fault vmf = {
-		.vma = vma,
-		.address = haddr,
-		.real_address = addr,
-		.flags = flags,
+		haddr = addr & PAGE_MASK;
 
-		/*
-		 * Hard to debug if it ends up being
-		 * used by a callee that assumes
-		 * something about the other
-		 * uninitialized fields... same as in
-		 * memory.c
-		 */
-	};
+	vmf.vma = vma;
+	vmf.address = haddr;
+	vmf.real_address = addr;
+	vmf.flags = flags;
+	/*
+	 * Hard to debug if it ends up being
+	 * used by a callee that assumes
+	 * something about the other
+	 * uninitialized fields... same as in
+	 * memory.c
+	 */
 
 	/*
 	 * vma_lock and hugetlb_fault_mutex must be dropped before handling
-- 
2.39.0.314.g84b9a713c41-goog