Re: [PATCH 1/2] mm: Warn once when a page is freed with PG_mlocked

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Apr 27, 2012 at 10:42:26AM -0700, Ying Han wrote:
> I am resending this patch orginally from Mel, and the reason we spotted this
> is due to the next patch where I am adding the mlock stat into per-memcg
> meminfo. We found out that it is impossible to update the counter if the page
> is in the freeing patch w/ mlocked bit set.
> 
> Then we started wondering if it is possible at all. It shouldn't happen that
> freeing a mlocked page without going through munlock_vma_pages_all(). Looks
> like it did happen few years ago, and here is the patch introduced it
> 
> commit 985737cf2ea096ea946aed82c7484d40defc71a8
> Author: Lee Schermerhorn <lee.schermerhorn@hp.com>
> Date:   Sat Oct 18 20:26:53 2008 -0700
> 
>     mlock: count attempts to free mlocked page

I was going to ask what changed that it can't happen anymore, but then
remembered how I was able to up this counter in the past: truncating a
private file mapping with mlocked COWed anon pages.  Because this path
unmaps vmas coming from the inode, it takes mapping->i_mmap_mutex, and
you can't nest page lock inside it to do the munlock (because of rmap).

> There are two ways to persue and I would like to ask people's opinion:
> 
> 1. revert the patch totally and the page will get into bad_page(). Then we
> get the report as well.
> 
> 2. fix up the page like the patch does but put on warn_once() to report the
> problem.
> 
> People might feel more confident by doing step by step which adding the
> warn_on() first and then revert it later. So I resend the patch from Mel and
> here is the patch:
> 
> When a page is freed with the PG_mlocked set, it is considered an unexpected
> but recoverable situation. A counter records how often this event happens
> but it is easy to miss that this event has occured at all. This patch warns
> once when PG_mlocked is set to prompt debuggers to check the counter to
> see how often it is happening.

Here is a program that will trigger your warning.

dexter:~$ grep mlockfreed /proc/vmstat 
unevictable_pgs_mlockfreed 3
dexter:~$ ./mlockfree 
dexter:~$ grep mlockfreed /proc/vmstat 
unevictable_pgs_mlockfreed 4

---

#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>

int main(void)
{
	char *map;
	int fd;

	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);
	return 0;
}

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