linux-kernel - Re: [PATCH 05/30] mm: slb: add knowledge of reserve pages

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Date:	Tue, 12 Aug 2008 19:35:50 +1000
From:	Neil Brown <neilb@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	netdev@...r.kernel.org, trond.myklebust@....uio.no,
	Daniel Lezcano <dlezcano@...ibm.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH 05/30] mm: slb: add knowledge of reserve pages

On Tuesday August 12, a.p.zijlstra@...llo.nl wrote:
> On Tue, 2008-08-12 at 15:35 +1000, Neil Brown wrote:
> > On Thursday July 24, a.p.zijlstra@...llo.nl wrote:
> > > Restrict objects from reserve slabs (ALLOC_NO_WATERMARKS) to allocation
> > > contexts that are entitled to it. This is done to ensure reserve pages don't
> > > leak out and get consumed.
> > 
> > This looks good (we are still missing slob though, aren't we :-( )
> 
> I actually have that now, just needs some testing..

Cool!

> 
> > > @@ -1526,7 +1540,7 @@ load_freelist:
> > >  	object = c->page->freelist;
> > >  	if (unlikely(!object))
> > >  		goto another_slab;
> > > -	if (unlikely(SLABDEBUG && PageSlubDebug(c->page)))
> > > +	if (unlikely(PageSlubDebug(c->page) || c->reserve))
> > >  		goto debug;
> > 
> > This looks suspiciously like debugging code that you have left in.
> > Is it??
> 
> Its not, we need to force slub into the debug slow path when we have a
> reserve page, otherwise we cannot do the permission check on each
> allocation.

I see.... a little.  I'm trying to avoid understanding slub too
deeply, I don't want to use up valuable brain cell :-)
Would we be justified in changing the label from 'debug:' to
'slow_path:'  or something?  And if it is just c->reserve, should
we avoid the call to alloc_debug_processing?


Thanks,
NeilBrown

> 
> > > @@ -265,7 +267,8 @@ struct array_cache {
> > >  	unsigned int avail;
> > >  	unsigned int limit;
> > >  	unsigned int batchcount;
> > > -	unsigned int touched;
> > > +	unsigned int touched:1,
> > > +		     reserve:1;
> > 
> > This sort of thing always worries me.
> > It is a per-cpu data structure so you won't get SMP races corrupting
> > fields.  But you do get read-modify-write in place of simple updates.
> > I guess it's not a problem..  But it worries me :-)
> 
> Right,.. do people prefer I just add another int?
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