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

Peter Zijlstra
2008-08-12T07:22:43Z

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..

> > @@ -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.

> > @@ -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?