linux-kernel - Re: a patch drop request in -mm

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Message-ID: <20090921152219.GQ12726@csn.ul.ie>
Date:	Mon, 21 Sep 2009 16:22:19 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: a patch drop request in -mm

On Tue, Sep 22, 2009 at 12:00:51AM +0900, KOSAKI Motohiro wrote:
> Mel,
> 
> Today, my test found following patch makes false-positive warning.
> because, truncate can free the pages
> although the pages are mlock()ed.
> 
> So, I think following patch should be dropped.
> .. or, do you think truncate should clear PG_mlock before free the page?
> 

Is there a reason that truncate cannot clear PG_mlock before freeing the
page?

> Can I ask your patch intention?


Locked pages being freed to the page allocator were considered
unexpected and a counter was in place to determine how often that
situation occurred. However, I considered it unlikely that the counter
would be noticed so the warning was put in place to catch what class of
pages were getting freed locked inappropriately. I think a few anomolies
have been cleared up since. Ultimately, it should have been safe to
delete the check.

> 
> 
> =============================================================
> commit 7a06930af46eb39351cbcdc1ab98701259f9a72c
> Author: Mel Gorman <mel@....ul.ie>
> Date:   Tue Aug 25 00:43:07 2009 +0200
> 
>     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.
> 
>     Signed-off-by: Mel Gorman <mel@....ul.ie>
>     Reviewed-by: Christoph Lameter <cl@...ux-foundation.org>
>     Acked-by: Johannes Weiner <hannes@...xchg.org>
>     Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 28c2f3e..251fd73 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -494,6 +494,11 @@ static inline void __free_one_page(struct page *page,
>   */
>  static inline void free_page_mlock(struct page *page)
>  {
> +       WARN_ONCE(1, KERN_WARNING
> +               "Page flag mlocked set for process %s at pfn:%05lx\n"
> +               "page:%p flags:%#lx\n",
> +               current->comm, page_to_pfn(page),
> +               page, page->flags|__PG_MLOCKED);
>         __dec_zone_page_state(page, NR_MLOCK);
>         __count_vm_event(UNEVICTABLE_MLOCKFREED);
>  }
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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