Re: Mlocked pages statistics shows bogus value.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Kirill A. Shutemov wrote:
> Oh. Looks like a bug from 2013...
> 
> Thanks for report.
> 
> From 6f80a79dc5f65f29899e396942d40f727cd36480 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Date: Tue, 19 Jan 2016 14:59:19 +0300
> Subject: [PATCH] mm: fix mlock accouting
> 
> 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 singed int.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Fixes: ff6a6da60b89 ("mm: accelerate munlock() treatment of THP pages")
> Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Michel Lespinasse <walken@google.com>
> ---
>  mm/mlock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mlock.c b/mm/mlock.c
> index e1e2b1207bf2..96f001041928 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -175,7 +175,7 @@ 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 */
> -- 
>  Kirill A. Shutemov
> 

Don't we want to use "long" than "int" for all variables that count number
of pages, for recently commit 6cdb18ad98a49f7e9b95d538a0614cde827404b8
"mm/vmstat: fix overflow in mod_zone_page_state()" changed to use "long" ?

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