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

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Message-Id: <20070628.212825.78710369.davem@davemloft.net>
Date:	Thu, 28 Jun 2007 21:28:25 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	clameter@....com
Cc:	hugh@...itas.com, James.Bottomley@...eleye.com,
	rmk+lkml@....linux.org.uk, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Containment measures for slab objects on scatter
 gather lists

From: Christoph Lameter <clameter@....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.
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