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>

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);
 		}