linux-kernel - Re: SLUB defrag pull request?

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Date:	Wed, 22 Oct 2008 09:10:36 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	cl@...ux-foundation.org
CC:	miklos@...redi.hu, 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 Tue, 21 Oct 2008, Christoph Lameter wrote:
> The only way that a secure reference can be established is if the
> slab page is locked. That requires a spinlock. The slab allocator
> calls the get() functions while the slab lock guarantees object
> existence. Then locks are dropped and reclaim actions can start with
> the guarantee that the slab object will not suddenly vanish.

Yes, you've made up your mind, that you want to do it this way.  But
it's the _wrong_ way, this "want to get a secure reference for use
later" leads to madness when applied to dentries or inodes.  Try for a
minute to think outside this template.

For example dcache_lock will protect against dentries moving to/from
d_lru.  So you can do this:

  take dcache_lock
  check if d_lru is non-empty
  take sb->s_umount
  free dentry
  release sb->s_umount
  release dcache_lock

Yeah, locking will be more complicated in reality.  Still, much less
complicated than trying to do the same across two separate phases.

Why can't something like that work?

Miklos
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