Re: Mlocked pages statistics shows bogus value.

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Jan 19, 2016 at 07:36:37PM +0900, Tetsuo Handa wrote:
> While reading OOM report from Jan Stancek, I noticed that
> NR_MLOCK statistics shows bogus values.
> 
> 
> Steps to reproduce:
> 
> (1) Check Mlocked: field of /proc/meminfo or mlocked: field of SysRq-m.
> 
> (2) Compile and run below program with appropriate size as argument.
>     There is no need to invoke the OOM killer.
> 
> ----------
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/mman.h>
> 
> int main(int argc, char *argv[])
> {
> 	unsigned long length = atoi(argv[1]);
> 	void *addr = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> 	if (addr == MAP_FAILED)
> 		printf("mmap() failed\n");
> 	else if (mlock(addr, length) == -1)
> 		printf("mlock() failed\n");
> 	else
> 		printf("MLocked %lu bytes\n", length);
> 	return 0;
> }
> ----------
> 
> (3) Check Mlocked: field or mlocked: field again.
>     You can see the value became very large due to
>     NR_MLOCK counter going negative.

Oh. Looks like a bug from 2013...

Thanks for report.