linux-kernel - Re: SLUB defrag pull request?

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Message-ID: <20081020230420.GC21152@disturbed>
Date:	Tue, 21 Oct 2008 10:04:20 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Miklos Szeredi <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 Mon, Oct 20, 2008 at 02:53:40PM -0500, Christoph Lameter wrote:
> Miklos Szeredi wrote:
> > Case below was brainfart, please ignore.  But that doesn't really
> > help: the VFS assumes that you cannot umount while there are busy
> > dentries/inodes.  Usually it works this way: VFS first gets vfsmount
> > ref, then gets dentry ref, and releases them in the opposite order.
> > And umount is not allowed if vfsmount has a non-zero refcount (it's a
> > bit more complicated, but the essense is the same).
> 
> The dentries that we get a ref on are candidates for removal. Their lifetime
> is limited. Unmounting while we are trying to remove dentries/inodes results
> in two mechanisms removing dentries/inodes.
> 
> If we have obtained a reference then invalidate_list() will return the number
> of busy inodes which would trigger the printk in generic_shutdown_super(). But
> these are inodes currently being reclaimed by slab defrag. Just waiting a bit
> would remedy the situation.
> 
> We would need some way to make generic_shutdown_super() wait until slab defrag
> is finished.

Seems to me that prune_dcache() handles this case by holding the sb->s_umount
semaphore while pruning. The same logic applies here, right?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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