Re: [PATCH 1/4] hugetlb: skip to end of PT page mapping when pte not present

On 06/17/22 10:15, Peter Xu wrote:

Hi, Mike,

On Thu, Jun 16, 2022 at 02:05:15PM -0700, Mike Kravetz wrote:

quoted

@@ -6877,6 +6896,39 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
 	return (pte_t *)pmd;
 }
 
+/*
+ * Return a mask that can be used to update an address to the last huge
+ * page in a page table page mapping size.  Used to skip non-present
+ * page table entries when linearly scanning address ranges.  Architectures
+ * with unique huge page to page table relationships can define their own
+ * version of this routine.
+ */
+unsigned long hugetlb_mask_last_page(struct hstate *h)
+{
+	unsigned long hp_size = huge_page_size(h);
+
+	switch (hp_size) {
+	case P4D_SIZE:
+		return PGDIR_SIZE - P4D_SIZE;
+	case PUD_SIZE:
+		return P4D_SIZE - PUD_SIZE;
+	case PMD_SIZE:
+		return PUD_SIZE - PMD_SIZE;
+	default:

Should we add a WARN_ON_ONCE() if it should never trigger?

Sure.  I will add this.

quoted

+		break; /* Should never happen */
+	}
+
+	return ~(0UL);
+}
+
+#else
+
+/* See description above.  Architectures can provide their own version. */
+__weak unsigned long hugetlb_mask_last_page(struct hstate *h)
+{
+	return ~(0UL);

I'm wondering whether it's better to return 0 rather than ~0 by default.
Could an arch with !CONFIG_ARCH_WANT_GENERAL_HUGETLB wrongly skip some
valid address ranges with ~0, or perhaps I misread?

Thank you, thank you, thank you Peter!

Yes, the 'default' return for hugetlb_mask_last_page() should be 0.  If
there is no 'optimization', we do not want to modify the address so we
want to OR with 0 not ~0.  My bad, I must have been thinking AND instead
of OR.

I will change here as well as in Baolin's patch.
-- 
Mike Kravetz

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