SHM_UNLOCK: fix Unevictable pages stranded after swap

Commit cc39c6a9bbde ("mm: account skipped entries to avoid looping in
find_get_pages") correctly fixed an infinite loop; but left a problem
that find_get_pages() on shmem would return 0 (appearing to callers to
mean end of tree) when it meets a run of nr_pages swap entries.

The only uses of find_get_pages() on shmem are via pagevec_lookup(),
called from invalidate_mapping_pages(), and from shmctl SHM_UNLOCK's
scan_mapping_unevictable_pages().  The first is already commented, and
not worth worrying about; but the second can leave pages on the
Unevictable list after an unusual sequence of swapping and locking.

Fix that by using shmem_find_get_pages_and_swap() (then ignoring the
swap) instead of pagevec_lookup().

But I don't want to contaminate vmscan.c with shmem internals, nor
shmem.c with LRU locking.  So move scan_mapping_unevictable_pages() into
shmem.c, renaming it shmem_unlock_mapping(); and rename
check_move_unevictable_page() to check_move_unevictable_pages(), looping
down an array of pages, oftentimes under the same lock.

Leave out the "rotate unevictable list" block: that's a leftover from
when this was used for /proc/sys/vm/scan_unevictable_pages, whose flawed
handling involved looking at pages at tail of LRU.

Was there significance to the sequence first ClearPageUnevictable, then
test page_evictable, then SetPageUnevictable here? I think not, we're
under LRU lock, and have no barriers between those.

Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Shaohua Li <shaohua.li@intel.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michel Lespinasse <walken@google.com>
Cc: <stable@vger.kernel.org> [back to 3.1 but will need respins]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 80 insertions, 94 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 4aaa53abe302..269d049294ab 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -379,7 +379,7 @@ static int shmem_free_swap(struct address_space *mapping,
 /*
  * Pagevec may contain swap entries, so shuffle up pages before releasing.
  */
-static void shmem_pagevec_release(struct pagevec *pvec)
+static void shmem_deswap_pagevec(struct pagevec *pvec)
 {
         int i, j;
 
@@ -389,7 +389,36 @@ static void shmem_pagevec_release(struct pagevec *pvec)
                         pvec->pages[j++] = page;
         }
         pvec->nr = j;
-        pagevec_release(pvec);
+}
+
+/*
+ * SysV IPC SHM_UNLOCK restore Unevictable pages to their evictable lists.
+ */
+void shmem_unlock_mapping(struct address_space *mapping)
+{
+        struct pagevec pvec;
+        pgoff_t indices[PAGEVEC_SIZE];
+        pgoff_t index = 0;
+
+        pagevec_init(&pvec, 0);
+        /*
+         * Minor point, but we might as well stop if someone else SHM_LOCKs it.
+         */
+        while (!mapping_unevictable(mapping)) {
+                /*
+                 * Avoid pagevec_lookup(): find_get_pages() returns 0 as if it
+                 * has finished, if it hits a row of PAGEVEC_SIZE swap entries.
+                 */
+                pvec.nr = shmem_find_get_pages_and_swap(mapping, index,
+                                        PAGEVEC_SIZE, pvec.pages, indices);
+                if (!pvec.nr)
+                        break;
+                index = indices[pvec.nr - 1] + 1;
+                shmem_deswap_pagevec(&pvec);
+                check_move_unevictable_pages(pvec.pages, pvec.nr);
+                pagevec_release(&pvec);
+                cond_resched();
+        }
 }
 
 /*
@@ -440,7 +469,8 @@ void shmem_truncate_range(struct inode *inode, loff_t lstart, loff_t lend)
                         }
                         unlock_page(page);
                 }
-                shmem_pagevec_release(&pvec);
+                shmem_deswap_pagevec(&pvec);
+                pagevec_release(&pvec);
                 mem_cgroup_uncharge_end();
                 cond_resched();
                 index++;
@@ -470,7 +500,8 @@ void shmem_truncate_range(struct inode *inode, loff_t lstart, loff_t lend)
                         continue;
                 }
                 if (index == start && indices[0] > end) {
-                        shmem_pagevec_release(&pvec);
+                        shmem_deswap_pagevec(&pvec);
+                        pagevec_release(&pvec);
                         break;
                 }
                 mem_cgroup_uncharge_start();
@@ -494,7 +525,8 @@ void shmem_truncate_range(struct inode *inode, loff_t lstart, loff_t lend)
                         }
                         unlock_page(page);
                 }
-                shmem_pagevec_release(&pvec);
+                shmem_deswap_pagevec(&pvec);
+                pagevec_release(&pvec);
                 mem_cgroup_uncharge_end();
                 index++;
         }
@@ -2438,6 +2470,10 @@ int shmem_lock(struct file *file, int lock, struct user_struct *user)
         return 0;
 }
 
+void shmem_unlock_mapping(struct address_space *mapping)
+{
+}
+
 void shmem_truncate_range(struct inode *inode, loff_t lstart, loff_t lend)
 {
         truncate_inode_pages_range(inode->i_mapping, lstart, lend);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index e097c1026b58..c52b23552659 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -26,7 +26,6 @@
 #include <linux/buffer_head.h>  /* for try_to_release_page(),
                                         buffer_heads_over_limit */
 #include <linux/mm_inline.h>
-#include <linux/pagevec.h>
 #include <linux/backing-dev.h>
 #include <linux/rmap.h>
 #include <linux/topology.h>
@@ -661,7 +660,7 @@ redo:
                  * When racing with an mlock or AS_UNEVICTABLE clearing
                  * (page is unlocked) make sure that if the other thread
                  * does not observe our setting of PG_lru and fails
-                 * isolation/check_move_unevictable_page,
+                 * isolation/check_move_unevictable_pages,
                  * we see PG_mlocked/AS_UNEVICTABLE cleared below and move
                  * the page back to the evictable list.
                  *
@@ -3501,107 +3500,58 @@ int page_evictable(struct page *page, struct vm_area_struct *vma)
 
 #ifdef CONFIG_SHMEM
 /**
- * check_move_unevictable_page - check page for evictability and move to appropriate zone lru list
- * @page: page to check evictability and move to appropriate lru list
- * @zone: zone page is in
+ * check_move_unevictable_pages - check pages for evictability and move to appropriate zone lru list
+ * @pages:      array of pages to check
+ * @nr_pages:   number of pages to check
  *
- * Checks a page for evictability and moves the page to the appropriate
- * zone lru list.
- *
- * Restrictions: zone->lru_lock must be held, page must be on LRU and must
- * have PageUnevictable set.
+ * Checks pages for evictability and moves them to the appropriate lru list.
  *
  * This function is only used for SysV IPC SHM_UNLOCK.
  */
-static void check_move_unevictable_page(struct page *page, struct zone *zone)
+void check_move_unevictable_pages(struct page **pages, int nr_pages)
 {
         struct lruvec *lruvec;
+        struct zone *zone = NULL;
+        int pgscanned = 0;
+        int pgrescued = 0;
+        int i;
 
-        VM_BUG_ON(PageActive(page));
-retry:
-        ClearPageUnevictable(page);
-        if (page_evictable(page, NULL)) {
-                enum lru_list l = page_lru_base_type(page);
-
-                __dec_zone_state(zone, NR_UNEVICTABLE);
-                lruvec = mem_cgroup_lru_move_lists(zone, page,
-                                                   LRU_UNEVICTABLE, l);
-                list_move(&page->lru, &lruvec->lists[l]);
-                __inc_zone_state(zone, NR_INACTIVE_ANON + l);
-                __count_vm_event(UNEVICTABLE_PGRESCUED);
-        } else {
-                /*
-                 * rotate unevictable list
-                 */
-                SetPageUnevictable(page);
-                lruvec = mem_cgroup_lru_move_lists(zone, page, LRU_UNEVICTABLE,
-                                                   LRU_UNEVICTABLE);
-                list_move(&page->lru, &lruvec->lists[LRU_UNEVICTABLE]);
-                if (page_evictable(page, NULL))
-                        goto retry;
-        }
-}
-
-/**
- * scan_mapping_unevictable_pages - scan an address space for evictable pages
- * @mapping: struct address_space to scan for evictable pages
- *
- * Scan all pages in mapping.  Check unevictable pages for
- * evictability and move them to the appropriate zone lru list.
- *
- * This function is only used for SysV IPC SHM_UNLOCK.
- */
-void scan_mapping_unevictable_pages(struct address_space *mapping)
-{
-        pgoff_t next = 0;
-        pgoff_t end   = (i_size_read(mapping->host) + PAGE_CACHE_SIZE - 1) >>
-                         PAGE_CACHE_SHIFT;
-        struct zone *zone;
-        struct pagevec pvec;
-
-        if (mapping->nrpages == 0)
-                return;
-
-        pagevec_init(&pvec, 0);
-        while (next < end &&
-                pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) {
-                int i;
-                int pg_scanned = 0;
-
-                zone = NULL;
-
-                for (i = 0; i < pagevec_count(&pvec); i++) {
-                        struct page *page = pvec.pages[i];
-                        pgoff_t page_index = page->index;
-                        struct zone *pagezone = page_zone(page);
+        for (i = 0; i < nr_pages; i++) {
+                struct page *page = pages[i];
+                struct zone *pagezone;
 
-                        pg_scanned++;
-                        if (page_index > next)
-                                next = page_index;
-                        next++;
+                pgscanned++;
+                pagezone = page_zone(page);
+                if (pagezone != zone) {
+                        if (zone)
+                                spin_unlock_irq(&zone->lru_lock);
+                        zone = pagezone;
+                        spin_lock_irq(&zone->lru_lock);
+                }
 
-                        if (pagezone != zone) {
-                                if (zone)
-                                        spin_unlock_irq(&zone->lru_lock);
-                                zone = pagezone;
-                                spin_lock_irq(&zone->lru_lock);
-                        }
+                if (!PageLRU(page) || !PageUnevictable(page))
+                        continue;
 
-                        if (PageLRU(page) && PageUnevictable(page))
-                                check_move_unevictable_page(page, zone);
+                if (page_evictable(page, NULL)) {
+                        enum lru_list lru = page_lru_base_type(page);
+
+                        VM_BUG_ON(PageActive(page));
+                        ClearPageUnevictable(page);
+                        __dec_zone_state(zone, NR_UNEVICTABLE);
+                        lruvec = mem_cgroup_lru_move_lists(zone, page,
+                                                LRU_UNEVICTABLE, lru);
+                        list_move(&page->lru, &lruvec->lists[lru]);
+                        __inc_zone_state(zone, NR_INACTIVE_ANON + lru);
+                        pgrescued++;
                 }
-                if (zone)
-                        spin_unlock_irq(&zone->lru_lock);
-                pagevec_release(&pvec);
+        }
 
-                count_vm_events(UNEVICTABLE_PGSCANNED, pg_scanned);
-                cond_resched();
+        if (zone) {
+                __count_vm_events(UNEVICTABLE_PGRESCUED, pgrescued);
+                __count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
+                spin_unlock_irq(&zone->lru_lock);
         }
 }
-#else
-void scan_mapping_unevictable_pages(struct address_space *mapping)
-{
-}
 #endif /* CONFIG_SHMEM */
 
 static void warn_scan_unevictable_pages(void)