[PATCH v4 4/7] mm/thp: Carry over dirty bit when thp splits on pmd

[Peter Xu]

Carry over the dirty bit from pmd to pte when a huge pmd splits.  It
shouldn't be a correctness issue since when pmd_dirty() we'll have the page
marked dirty anyway, however having dirty bit carried over helps the next
initial writes of split ptes on some archs like x86.

Reviewed-by: Huang Ying <ying.huang@xxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 mm/huge_memory.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3222b40a0f6d..2f68e034ddec 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2027,7 +2027,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 	pgtable_t pgtable;
 	pmd_t old_pmd, _pmd;
 	bool young, write, soft_dirty, pmd_migration = false, uffd_wp = false;
-	bool anon_exclusive = false;
+	bool anon_exclusive = false, dirty = false;
 	unsigned long addr;
 	int i;
 
@@ -2116,8 +2116,10 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 		uffd_wp = pmd_swp_uffd_wp(old_pmd);
 	} else {
 		page = pmd_page(old_pmd);
-		if (pmd_dirty(old_pmd))
+		if (pmd_dirty(old_pmd)) {
+			dirty = true;
 			SetPageDirty(page);
+		}
 		write = pmd_write(old_pmd);
 		young = pmd_young(old_pmd);
 		soft_dirty = pmd_soft_dirty(old_pmd);
@@ -2183,6 +2185,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 				entry = pte_wrprotect(entry);
 			if (!young)
 				entry = pte_mkold(entry);
+			/* NOTE: this may set soft-dirty too on some archs */
+			if (dirty)
+				entry = pte_mkdirty(entry);
 			if (soft_dirty)
 				entry = pte_mksoft_dirty(entry);
 			if (uffd_wp)
-- 
2.32.0