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 propertly of pagecache: an mlocked unmapped page is nonsensical.
Reported-by: Johannes Weiner <han...@cmpxchg.org>
Signed-off-by: Hugh Dickins <hu...@google.com>
Cc: Mel Gorman <m...@csn.ul.ie>
Cc: Rik van Riel <r...@redhat.com>
Cc: Michel Lespinasse <wal...@google.com>
Cc: Ying Han <ying...@google.com>
---
mm/internal.h | 7 +------
mm/memory.c | 10 +++++-----
mm/mlock.c | 16 +++-------------
mm/rmap.c | 4 ++++
mm/truncate.c | 4 ----
5 files changed, 13 insertions(+), 28 deletions(-)
--- 3.6-rc6.orig/mm/internal.h 2012-09-18 16:39:50.000000000 -0700
+++ 3.6-rc6/mm/internal.h 2012-09-18 17:51:02.871288773 -0700
@@ -200,12 +200,7 @@ extern void munlock_vma_page(struct 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
--- 3.6-rc6.orig/mm/memory.c 2012-09-18 15:38:08.000000000 -0700
+++ 3.6-rc6/mm/memory.c 2012-09-18 17:51:02.871288773 -0700
@@ -1576,12 +1576,12 @@ 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);
}
}
--- 3.6-rc6.orig/mm/mlock.c 2012-09-18 15:38:08.000000000 -0700
+++ 3.6-rc6/mm/mlock.c 2012-09-18 17:51:02.871288773 -0700
@@ -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_a
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);
}
--- 3.6-rc6.orig/mm/rmap.c 2012-09-18 16:39:50.000000000 -0700
+++ 3.6-rc6/mm/rmap.c 2012-09-18 17:51:02.871288773 -0700
@@ -1203,7 +1203,10 @@ 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
@@ -1213,6 +1216,7 @@ 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);
--- 3.6-rc6.orig/mm/truncate.c 2012-09-18 15:42:17.000000000 -0700
+++ 3.6-rc6/mm/truncate.c 2012-09-18 17:51:02.875288902 -0700
@@ -107,7 +107,6 @@ truncate_complete_page(struct address_sp
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_
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
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;
--
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