linux-kernel - Re: [PATCH] mm: Warn once when a page is freed with PG

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Message-ID: <20090715124509.GA1854@cmpxchg.org>
Date:	Wed, 15 Jul 2009 14:45:09 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	kosaki.motohiro@...fujitsu.com,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Jiri Slaby <jirislaby@...il.com>
Subject: Re: [PATCH] mm: Warn once when a page is freed with PG_mlocked set (resend)

Hello Mel,

On Wed, Jul 15, 2009 at 11:48:34AM +0100, Mel Gorman wrote:
> 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>
> --- 
>  mm/page_alloc.c |   16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index caa9268..f8902e7 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -495,8 +495,16 @@ static inline void free_page_mlock(struct page *page)
>  static void free_page_mlock(struct page *page) { }
>  #endif
>  
> -static inline int free_pages_check(struct page *page)
> -{
> +static inline int free_pages_check(struct page *page, int wasMlocked)
> +{
> +	if (unlikely(wasMlocked)) {
> +		WARN_ONCE(1, KERN_WARNING
> +			"Page flag mlocked set for process %s at pfn:%05lx\n"
> +			"page:%p flags:0x%lX\n",
> +			current->comm, page_to_pfn(page),
> +			page, page->flags|__PG_MLOCKED);

Since the warning is the only action in this branch, wouldn't
WARN_ONCE(wasMlocked, KERN_WARNING ...) be better?

	Hannes
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