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

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Date:	Tue, 28 Apr 2009 16:42:48 -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 18:31:09 -0500
Matt Mackall <mpm@...enic.com> wrote:

> On Tue, 2009-04-28 at 16:02 -0700, Andrew Morton wrote:
> > 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?
> 
> As I mentioned just a bit ago, it's really not an unreasonable use case
> to want to do this on every page in the system back to back. So per page
> overhead matters. And the odds of stalling on a locked page when
> visiting 1M pages while under load are probably not negligible.

The chances of stalling on a locked page are pretty good, and the
duration of the stall might be long indeed.  Perhaps a trylock is a
decent compromise - it depends on the value of this metric, and I've
forgotten what we're talking about ;)

umm, seems that this flag is needed to enable PG_error, PG_dirty,
PG_uptodate and PG_writeback reporting.  So simply removing this code
would put a huge hole in the patchset, no?

> Our lock primitives are pretty low overhead in the fast path, but every
> cycle counts. The new tests and branches this code already adds are a
> bit worrisome, but on balance probably worth it.

That should be easy to quantify (hint).
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