mm: use clear_page_mlock() in page_remove_rmap()

We had thought that pages could no longer get freed while still marked as
mlocked; but Johannes Weiner posted this program to demonstrate that
truncating an mlocked private file mapping containing COWed pages is still
mishandled:

#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>

int main(void)
{
	char *map;
	int fd;

	system("grep mlockfreed /proc/vmstat");
	fd = open("chigurh", O_CREAT|O_EXCL|O_RDWR);
	unlink("chigurh");
	ftruncate(fd, 4096);
	map = mmap(NULL, 4096, PROT_WRITE, MAP_PRIVATE, fd, 0);
	map[0] = 11;
	mlock(map, sizeof(fd));
	ftruncate(fd, 0);
	close(fd);
	munlock(map, sizeof(fd));
	munmap(map, 4096);
	system("grep mlockfreed /proc/vmstat");
	return 0;
}

The anon COWed pages are not caught by truncation's clear_page_mlock() of
the pagecache pages; but unmap_mapping_range() unmaps them, so we ought to
look out for them there in page_remove_rmap().  Indeed, why should
truncation or invalidation be doing the clear_page_mlock() when removing
from pagecache?  mlock is a property of mapping in userspace, not a
property of pagecache: an mlocked unmapped page is nonsensical.

Reported-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Rik van Riel <riel@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Ying Han <yinghan@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

5 files changed, 13 insertions, 28 deletions

diff --git a/mm/internal.h b/mm/internal.h
index 78f25d6cc6a7..4dc93e2fe69e 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -201,12 +201,7 @@ extern void munlock_vma_page(struct page *page);
  * If called for a page that is still mapped by mlocked vmas, all we do
  * is revert to lazy LRU behaviour -- semantics are not broken.
  */
-extern void __clear_page_mlock(struct page *page);
-static inline void clear_page_mlock(struct page *page)
-{
-        if (unlikely(TestClearPageMlocked(page)))
-                __clear_page_mlock(page);
-}
+extern void clear_page_mlock(struct page *page);
 
 /*
  * mlock_migrate_page - called only from migrate_page_copy() to
diff --git a/mm/memory.c b/mm/memory.c
index d205e4381a34..5f5d1f039bf4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1577,12 +1577,12 @@ split_fallthrough:
                 if (page->mapping && trylock_page(page)) {
                         lru_add_drain();  /* push cached pages to LRU */
                         /*
-                         * Because we lock page here and migration is
-                         * blocked by the pte's page reference, we need
-                         * only check for file-cache page truncation.
+                         * Because we lock page here, and migration is
+                         * blocked by the pte's page reference, and we
+                         * know the page is still mapped, we don't even
+                         * need to check for file-cache page truncation.
                          */
-                        if (page->mapping)
-                                mlock_vma_page(page);
+                        mlock_vma_page(page);
                         unlock_page(page);
                 }
         }
diff --git a/mm/mlock.c b/mm/mlock.c
index a948be4b7ba7..de7321592897 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -51,13 +51,10 @@ EXPORT_SYMBOL(can_do_mlock);
 /*
  *  LRU accounting for clear_page_mlock()
  */
-void __clear_page_mlock(struct page *page)
+void clear_page_mlock(struct page *page)
 {
-        VM_BUG_ON(!PageLocked(page));
-
-        if (!page->mapping) {   /* truncated ? */
+        if (!TestClearPageMlocked(page))
                 return;
-        }
 
         dec_zone_page_state(page, NR_MLOCK);
         count_vm_event(UNEVICTABLE_PGCLEARED);
@@ -290,14 +287,7 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
                 page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
                 if (page && !IS_ERR(page)) {
                         lock_page(page);
-                        /*
-                         * Like in __mlock_vma_pages_range(),
-                         * because we lock page here and migration is
-                         * blocked by the elevated reference, we need
-                         * only check for file-cache page truncation.
-                         */
-                        if (page->mapping)
-                                munlock_vma_page(page);
+                        munlock_vma_page(page);
                         unlock_page(page);
                         put_page(page);
                 }
diff --git a/mm/rmap.c b/mm/rmap.c
index 0d86433e42d7..bf03149f495c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1155,7 +1155,10 @@ void page_remove_rmap(struct page *page)
         } else {
                 __dec_zone_page_state(page, NR_FILE_MAPPED);
                 mem_cgroup_dec_page_stat(page, MEMCG_NR_FILE_MAPPED);
+                mem_cgroup_end_update_page_stat(page, &locked, &flags);
         }
+        if (unlikely(PageMlocked(page)))
+                clear_page_mlock(page);
         /*
          * It would be tidy to reset the PageAnon mapping here,
          * but that might overwrite a racing page_add_anon_rmap
@@ -1165,6 +1168,7 @@ void page_remove_rmap(struct page *page)
          * Leaving it set also helps swapoff to reinstate ptes
          * faster for those pages still in swapcache.
          */
+        return;
 out:
         if (!anon)
                 mem_cgroup_end_update_page_stat(page, &locked, &flags);
diff --git a/mm/truncate.c b/mm/truncate.c
index f38055cb8af6..d51ce92d6e83 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -107,7 +107,6 @@ truncate_complete_page(struct address_space *mapping, struct page *page)
 
         cancel_dirty_page(page, PAGE_CACHE_SIZE);
 
-        clear_page_mlock(page);
         ClearPageMappedToDisk(page);
         delete_from_page_cache(page);
         return 0;
@@ -132,7 +131,6 @@ invalidate_complete_page(struct address_space *mapping, struct page *page)
         if (page_has_private(page) && !try_to_release_page(page, 0))
                 return 0;
 
-        clear_page_mlock(page);
         ret = remove_mapping(mapping, page);
 
         return ret;
@@ -394,8 +392,6 @@ invalidate_complete_page2(struct address_space *mapping, struct page *page)
         if (page_has_private(page) && !try_to_release_page(page, GFP_KERNEL))
                 return 0;
 
-        clear_page_mlock(page);
-
         spin_lock_irq(&mapping->tree_lock);
         if (PageDirty(page))
                 goto failed;