Re: [PATCH] mm: Warn once when a page is freed with PG_mlocked set V2

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 23 Jul 2009 11:29:39 +0100 Mel Gorman <mel@csn.ul.ie> wrote:

> On Wed, Jul 22, 2009 at 04:06:49PM -0700, Andrew Morton wrote:
> > On Wed, 15 Jul 2009 10:31:54 -0400 (EDT)
> > Christoph Lameter <cl@linux-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.
> > 
> 
> It adds code duplication then, one of which is in a fast path.
> 
> > As it stands, I'm really doubting that the patch's utility is worth its
> > cost.
> > 
> 
> I'm happy to let this one drop. It seemed like it would be nice for debugging
> while there are still corner cases where mlocked pages are getting freed
> instead of torn down but we already account for that situation occuring. While
> I think it'll be tricky to spot, it's probably preferable to having warnings
> spew out onto dmesg.

If we do in it the way which Christoph recommends, the additional
overhead is miniscule?