[added to the 4.1 stable tree] mm: fix mlock accouting

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From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>

This patch has been added to the 4.1 stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit 7162a1e87b3e380133dadc7909081bb70d0a7041 ]

Tetsuo Handa reported underflow of NR_MLOCK on munlock.

Testcase:

    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/mman.h>

    #define BASE ((void *)0x400000000000)
    #define SIZE (1UL << 21)

    int main(int argc, char *argv[])
    {
        void *addr;

        system("grep Mlocked /proc/meminfo");
        addr = mmap(BASE, SIZE, PROT_READ | PROT_WRITE,
                MAP_ANONYMOUS | MAP_PRIVATE | MAP_LOCKED | MAP_FIXED,
                -1, 0);
        if (addr == MAP_FAILED)
            printf("mmap() failed\n"), exit(1);
        munmap(addr, SIZE);
        system("grep Mlocked /proc/meminfo");
        return 0;
    }

It happens on munlock_vma_page() due to unfortunate choice of nr_pages
data type:

    __mod_zone_page_state(zone, NR_MLOCK, -nr_pages);

For unsigned int nr_pages, implicitly casted to long in
__mod_zone_page_state(), it becomes something around UINT_MAX.

munlock_vma_page() usually called for THP as small pages go though
pagevec.

Let's make nr_pages signed int.

Similar fixes in 6cdb18ad98a4 ("mm/vmstat: fix overflow in
mod_zone_page_state()") used `long' type, but `int' here is OK for a
count of the number of sub-pages in a huge page.

Fixes: ff6a6da60b89 ("mm: accelerate munlock() treatment of THP pages")
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Tested-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Michel Lespinasse <walken@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>  [4.4+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
---
 mm/mlock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index 6fd2cf1..3d3ee6c 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -172,7 +172,7 @@ static void __munlock_isolation_failed(struct page *page)
  */
 unsigned int munlock_vma_page(struct page *page)
 {
-	unsigned int nr_pages;
+	int nr_pages;
 	struct zone *zone = page_zone(page);
 
 	/* For try_to_munlock() and to serialize with page migration */
-- 
2.5.0

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