Re: [PATCH] Containment measures for slab objects on scatter gather lists

[David Miller <davem@davemloft.net>]

From: Christoph Lameter <clameter@sgi.com>
Date: Thu, 28 Jun 2007 21:22:22 -0700 (PDT)

> On Thu, 28 Jun 2007, David Miller wrote:
> 
> > > Still a better solution would be to not use the slab allocator at all for 
> > > the objects that are used to send commands to the devices. These are not 
> > > permanent and grabbing a page from the pcp lists and putting it back is 
> > > likely as fast as performing a kmalloc.
> > 
> > Jens Axboe wants to get references to the page structs behind
> > kmalloc() allocated pages in his networking splice work.
> 
> You can get such a reference and then the slab page will be in limbo if 
> all objects are freed until that reference is given up. The reference 
> method is also use by kmem_cache_vacate() (but that is slab internal).

What about if someone kfree()'s that object meanwhile?

Can we bump the SLAB object count just like we can a page?
That's basically what's happening in the stuff Jens is
working on, he needs to grab a reference to a SLAB
object just like one can a page.  Even if there is an
intervening kfree (like a put_page()) the SLAB object is
still live until all the references are put, and thus it
can't get reallocated and given to another client by SLAB.