linux-kernel - Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

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Date:	Tue, 28 Apr 2009 16:02:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Matt Mackall <mpm@...enic.com>
Cc:	fengguang.wu@...el.com, linux-kernel@...r.kernel.org,
	kosaki.motohiro@...fujitsu.com, andi@...stfloor.org,
	adobriyan@...il.com, linux-mm@...ck.org
Subject: Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

On Tue, 28 Apr 2009 17:46:34 -0500
Matt Mackall <mpm@...enic.com> wrote:

> > > +/* a helper function _not_ intended for more general uses */
> > > +static inline int page_cap_writeback_dirty(struct page *page)
> > > +{
> > > +	struct address_space *mapping;
> > > +
> > > +	if (!PageSlab(page))
> > > +		mapping = page_mapping(page);
> > > +	else
> > > +		mapping = NULL;
> > > +
> > > +	return mapping && mapping_cap_writeback_dirty(mapping);
> > > +}
> > 
> > If the page isn't locked then page->mapping can be concurrently removed
> > and freed.  This actually happened to me in real-life testing several
> > years ago.
> 
> We certainly don't want to be taking locks per page to build the flags
> data here. As we don't have any pretense of being atomic, it's ok if we
> can find a way to do the test that's inaccurate when a race occurs, so
> long as it doesn't dereference null.
> 
> But if there's not an obvious way to do that, we should probably just
> drop this flag bit for this iteration.

trylock_page() could be used here, perhaps.

Then again, why _not_ just do lock_page()?  After all, few pages are
ever locked.  There will be latency if the caller stumbles across a
page which is under read I/O, but so be it?

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