Re: f45ec5ff16 ("userfaultfd: wp: support swap and page migration"): [ 140.777858] BUG: Bad rss-counter state mm:b278fc66 type:MM_ANONPAGES val:1

[Hillf Danton <hdanton@sina.com>]

On Fri, 10 Apr 2020 11:32:34 -0400 Peter Xu wrote:
> 
> I'm not sure this is correct.  As I mentioned, the commit wanted to
> apply the uffd-wp bit even for the swap entries so that even the swap
> entries got swapped in, the page will still be write protected.  So
> IIUC think we can't remove that.

Yes you are right.

Now both CONFIG_MIGRATION and swap entry are restored after making uffd_wq
survive migrate the same way as soft_dirty.

--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -236,6 +236,8 @@ static bool remove_migration_pte(struct
 		pte = pte_mkold(mk_pte(new, READ_ONCE(vma->vm_page_prot)));
 		if (pte_swp_soft_dirty(*pvmw.pte))
 			pte = pte_mksoft_dirty(pte);
+		if (pte_swp_uffd_wp(*pvmw.pte))
+			pte = pte_mkuffd_wp(pte);
 
 		/*
 		 * Recheck VMA as permissions can change since migration started
@@ -243,15 +245,11 @@ static bool remove_migration_pte(struct
 		entry = pte_to_swp_entry(*pvmw.pte);
 		if (is_write_migration_entry(entry))
 			pte = maybe_mkwrite(pte, vma);
-		else if (pte_swp_uffd_wp(*pvmw.pte))
-			pte = pte_mkuffd_wp(pte);
 
 		if (unlikely(is_zone_device_page(new))) {
 			if (is_device_private_page(new)) {
 				entry = make_device_private_entry(new, pte_write(pte));
 				pte = swp_entry_to_pte(entry);
-				if (pte_swp_uffd_wp(*pvmw.pte))
-					pte = pte_mkuffd_wp(pte);
 			}
 		}
 
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -139,11 +139,13 @@ static unsigned long change_pte_range(st
 			}
 			ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
 			pages++;
-		} else if (is_swap_pte(oldpte)) {
+		} else if (IS_ENABLED(CONFIG_MIGRATION)) {
 			swp_entry_t entry = pte_to_swp_entry(oldpte);
 			pte_t newpte;
 
-			if (is_write_migration_entry(entry)) {
+			if (!non_swap_entry(entry)) {
+				newpte = oldpte;
+			} else if (is_write_migration_entry(entry)) {
 				/*
 				 * A protection check is difficult so
 				 * just be safe and disable write
@@ -164,7 +166,7 @@ static unsigned long change_pte_range(st
 				if (pte_swp_uffd_wp(oldpte))
 					newpte = pte_swp_mkuffd_wp(newpte);
 			} else {
-				newpte = oldpte;
+				continue;
 			}
 
 			if (uffd_wp)