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

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Date:	Wed, 22 Jul 2009 16:06:49 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	mel@....ul.ie, kosaki.motohiro@...fujitsu.com,
	maximlevitsky@...il.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Lee.Schermerhorn@...com,
	penberg@...helsinki.fi, hannes@...xchg.org, jirislaby@...il.com
Subject: Re: [PATCH] mm: Warn once when a page is freed with PG_mlocked set
 V2

On Wed, 15 Jul 2009 10:31:54 -0400 (EDT)
Christoph Lameter <cl@...ux-foundation.org> wrote:

> On Wed, 15 Jul 2009, Mel Gorman wrote:
> 
> > -static inline int free_pages_check(struct page *page)
> > +static inline int free_pages_check(struct page *page, int wasMlocked)
> >  {
> > +	WARN_ONCE(wasMlocked, 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);
> > +
> >  	if (unlikely(page_mapcount(page) |
> 
> There is already a free_page_mlocked() that is only called if the mlock
> bit is set. Move it into there to avoid having to run two checks in the
> hot codee path?

Agreed.

This patch gratuitously adds hotpath overhead.  Moving the change to be
inside those preexisting wasMlocked tests will reduce its overhead a lot.

As it stands, I'm really doubting that the patch's utility is worth its
cost.

Also, it's a bit of a figleaf, but please consider making more use of
CONFIG_DEBUG_VM (see VM_BUG_ON()).

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