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

Neil Brown
2008-08-12T09:36:13Z

On Tuesday August 12, a.p.zijlstra@chello.nl wrote:
> On Tue, 2008-08-12 at 15:35 +1000, Neil Brown wrote:
> > On Thursday July 24, a.p.zijlstra@chello.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?