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mm: use clear_page_mlock() in page_remove_rmap()

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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 <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Ying Han <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Hugh Dickins 2012-10-08 16:33:19 -07:00 • committed by Linus Torvalds
commit e6c509f854
5 changed files with 13 additions and 28 deletions
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16
mm/mlock.c
View file

@ -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);
}