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

j.glisse
2013-08-12T15:43:32Z

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