[PATCH 07/10] mm: numa: Do not trap faults on the huge zero page

[Mel Gorman <mgorman@xxxxxxx>]

Faults on the huge zero page are pointless and there is a BUG_ON
to catch them during fault time. This patch reintroduces a check
that avoids marking the zero page PAGE_NONE.

Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 include/linux/huge_mm.h |  3 ++-
 mm/huge_memory.c        | 13 ++++++++++++-
 mm/memory.c             |  1 -
 mm/mprotect.c           | 15 ++++++++++++++-
 4 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 554bbe3..ad9051b 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -31,7 +31,8 @@ extern int move_huge_pmd(struct vm_area_struct *vma,
 			 unsigned long new_addr, unsigned long old_end,
 			 pmd_t *old_pmd, pmd_t *new_pmd);
 extern int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
-			unsigned long addr, pgprot_t newprot);
+			unsigned long addr, pgprot_t newprot,
+			int prot_numa);
 
 enum transparent_hugepage_flag {
 	TRANSPARENT_HUGEPAGE_FLAG,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 668f1a3..3013eb8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1502,7 +1502,7 @@ out:
  *  - HPAGE_PMD_NR is protections changed and TLB flush necessary
  */
 int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
-		unsigned long addr, pgprot_t newprot)
+		unsigned long addr, pgprot_t newprot, int prot_numa)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	spinlock_t *ptl;
@@ -1510,6 +1510,17 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 
 	if (__pmd_trans_huge_lock(pmd, vma, &ptl) == 1) {
 		pmd_t entry;
+
+		/*
+		 * Avoid trapping faults against the zero page. The read-only
+		 * data is likely to be read-cached on the local CPU and
+		 * local/remote hits to the zero page are not interesting.
+		 */
+		if (prot_numa && is_huge_zero_pmd(*pmd)) {
+			spin_unlock(ptl);
+			return 0;
+		}
+
 		ret = 1;
 		entry = pmdp_get_and_clear(mm, addr, pmd);
 		entry = pmd_modify(entry, newprot);
diff --git a/mm/memory.c b/mm/memory.c
index 900127b..a725c08 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3142,7 +3142,6 @@ static int do_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		pte_unmap_unlock(ptep, ptl);
 		return 0;
 	}
-	BUG_ON(is_zero_pfn(page_to_pfn(page)));
 
 	/*
 	 * Avoid grouping on DSO/COW pages in specific and RO pages
diff --git a/mm/mprotect.c b/mm/mprotect.c
index dc65c0f..33dfafb 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -75,6 +75,19 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 		oldpte = *pte;
 		if (pte_present(oldpte)) {
 			pte_t ptent;
+
+			/*
+			 * Avoid trapping faults against the zero or KSM
+			 * pages. See similar comment in change_huge_pmd.
+			 */
+			if (prot_numa) {
+				struct page *page;
+
+				page = vm_normal_page(vma, addr, oldpte);
+				if (!page || PageKsm(page))
+					continue;
+			}
+
 			ptent = ptep_modify_prot_start(mm, addr, pte);
 			ptent = pte_modify(ptent, newprot);
 
@@ -141,7 +154,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
 				split_huge_page_pmd(vma, addr, pmd);
 			else {
 				int nr_ptes = change_huge_pmd(vma, pmd, addr,
-						newprot);
+						newprot, prot_numa);
 
 				if (nr_ptes) {
 					if (nr_ptes == HPAGE_PMD_NR) {
-- 
2.1.2

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