[PATCH] mm: fix special swap entry handling on copy mm

[j.glisse@gmail.com]

From: Jerome Glisse <jglisse@redhat.com>

Prior to this copy_one_pte will never reach the special swap file
handling code because swap_duplicate will return invalid value.

Note this is not fatal so nothing bad ever happen because of that.
Reason is that copy_pte_range would break of its loop and call
add_swap_count_continuation which would see its a special swap
file and return 0 triggering copy_pte_range to try again. Because
we try again there is a huge chance that the temporarily special
migration pte is now again valid and pointing to a new valid page.

This patch just split handling of special swap entry from regular
one inside copy_one_pte.

(Note i spotted that while reading code i haven't tested my theory.)

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
---
 mm/memory.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 1ce2e2a..9f907dd 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -833,20 +833,20 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		if (!pte_file(pte)) {
 			swp_entry_t entry = pte_to_swp_entry(pte);
 
-			if (swap_duplicate(entry) < 0)
-				return entry.val;
-
-			/* make sure dst_mm is on swapoff's mmlist. */
-			if (unlikely(list_empty(&dst_mm->mmlist))) {
-				spin_lock(&mmlist_lock);
-				if (list_empty(&dst_mm->mmlist))
-					list_add(&dst_mm->mmlist,
-						 &src_mm->mmlist);
-				spin_unlock(&mmlist_lock);
-			}
-			if (likely(!non_swap_entry(entry)))
+			if (likely(!non_swap_entry(entry))) {
+				if (swap_duplicate(entry) < 0)
+					return entry.val;
+
+				/* make sure dst_mm is on swapoff's mmlist. */
+				if (unlikely(list_empty(&dst_mm->mmlist))) {
+					spin_lock(&mmlist_lock);
+					if (list_empty(&dst_mm->mmlist))
+						list_add(&dst_mm->mmlist,
+							 &src_mm->mmlist);
+					spin_unlock(&mmlist_lock);
+				}
 				rss[MM_SWAPENTS]++;
-			else if (is_migration_entry(entry)) {
+			} else if (is_migration_entry(entry)) {
 				page = migration_entry_to_page(entry);
 
 				if (PageAnon(page))
-- 
1.8.3.1

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