[PATCH v2 1/2] mm: Enable khugepaged anonymous collapse on non-writable regions

[Dev Jain <dev.jain@arm.com>]

Currently khugepaged does not collapse an anonymous region which does not
have a single writable pte. This is wasteful since a region mapped with
non-writable ptes, for example, non-writable VMAs mapped by the
application, won't benefit from THP collapse.

An additional consequence of this constraint is that MADV_COLLAPSE does not
perform a collapse on a non-writable VMA, and this restriction is nowhere
to be found on the manpage - the restriction itself sounds wrong to me
since the user knows the protection of the memory it has mapped, so
collapsing read-only memory via madvise() should be a choice of the
user which shouldn't be overridden by the kernel.

Therefore, remove this restriction by not honouring SCAN_PAGE_RO.

Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Zi Yan <ziy@nvidia.com> 
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 mm/khugepaged.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 4ec324a4c1fe..a0f1df2a7ae6 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -676,9 +676,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 			writable = true;
 	}
 
-	if (unlikely(!writable)) {
-		result = SCAN_PAGE_RO;
-	} else if (unlikely(cc->is_khugepaged && !referenced)) {
+	if (unlikely(cc->is_khugepaged && !referenced)) {
 		result = SCAN_LACK_REFERENCED_PAGE;
 	} else {
 		result = SCAN_SUCCEED;
@@ -1421,9 +1419,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
 		     mmu_notifier_test_young(vma->vm_mm, _address)))
 			referenced++;
 	}
-	if (!writable) {
-		result = SCAN_PAGE_RO;
-	} else if (cc->is_khugepaged &&
+	if (cc->is_khugepaged &&
 		   (!referenced ||
 		    (unmapped && referenced < HPAGE_PMD_NR / 2))) {
 		result = SCAN_LACK_REFERENCED_PAGE;
@@ -2830,7 +2826,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 		case SCAN_PMD_NULL:
 		case SCAN_PTE_NON_PRESENT:
 		case SCAN_PTE_UFFD_WP:
-		case SCAN_PAGE_RO:
 		case SCAN_LACK_REFERENCED_PAGE:
 		case SCAN_PAGE_NULL:
 		case SCAN_PAGE_COUNT:
-- 
2.30.2