linux-kernel - Re: SLUB defrag pull request?

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Date:	Wed, 22 Oct 2008 12:54:30 -0700 (PDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Miklos Szeredi <miklos@...redi.hu>
cc:	penberg@...helsinki.fi, nickpiggin@...oo.com.au, hugh@...itas.com,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: SLUB defrag pull request?

On Wed, 22 Oct 2008, Miklos Szeredi wrote:

> Why?  The kmem_cache_free() doesn't touch the contents of the object,
> does it?

Because filesystem code may be running on other processors which may be 
freeing the dentry.

>> Because the slab starts out with a series of objects left in a slab. It
>> needs to do build a list of objects etc in a way that is independent as
>> possible from the user of the slab page. It does that by locking the slab
>> page so that free operations stall until the reference has been
>> established. If it would not be shutting off frees then the objects could
>> vanish under us.
>
> It doesn't matter.  All we care about is that the dentry is on the
> lru: it's cached but unused.  Every other state (being created,
> active, being freed, freed) is uninteresting.

We cannot figure out that it is on the lru if we do not have a stable 
reference to the object.

> Sure, and all that is possible without doing this messy 2 phase thing.
> Unless I'm still missing something obvious...

Obviously one cannot free or handle an object that may be concurrently 
freed on another processor.

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