This is a helper macro to loop through all the usable page sizes for a
high-granularity-enabled HugeTLB VMA. Given the VMA's hstate, it will
loop, in descending order, through the page sizes that HugeTLB supports
for this architecture. It always includes PAGE_SIZE.

This is done by looping through the hstates; however, there is no
hstate for PAGE_SIZE. To handle this case, the loop intentionally goes
out of bounds, and the out-of-bounds pointer is mapped to PAGE_SIZE.

Signed-off-by: James Houghton <jthoughton@google.com>
---
 mm/hugetlb.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d6f07968156c..6eaec40d66ad 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -7856,6 +7856,25 @@ int enable_hugetlb_hgm(struct vm_area_struct *vma)
 	hugetlb_unshare_all_pmds(vma);
 	return 0;
 }
+
+/* Should only be used by the for_each_hgm_shift macro. */
+static unsigned int __shift_for_hstate(struct hstate *h)
+{
+	/* If h is out of bounds, we have reached the end, so give PAGE_SIZE */
+	if (h >= &hstates[hugetlb_max_hstate])
+		return PAGE_SHIFT;
+	return huge_page_shift(h);
+}
+
+/*
+ * Intentionally go out of bounds. An out-of-bounds hstate will be converted to
+ * PAGE_SIZE.
+ */
+#define for_each_hgm_shift(hstate, tmp_h, shift) \
+	for ((tmp_h) = hstate; (shift) = __shift_for_hstate(tmp_h), \
+			       (tmp_h) <= &hstates[hugetlb_max_hstate]; \
+			       (tmp_h)++)
+
 #endif /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */
 
 /*
-- 
2.38.0.135.g90850a2211-goog