linux-kernel - Re: SLUB defrag pull request?

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Date:	Mon, 20 Oct 2008 13:22:42 -0500
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?

Miklos Szeredi wrote:
> On Mon, 20 Oct 2008, Christoph Lameter wrote:
>>>> kick_inodes() only works on inodes that first have undergone 
>>>> get_inodes() where we establish a refcount under inode_lock(). The final 
>>>> cleanup in kick_inodes() is done under iprune_mutex. You are looking at 
>>>> the loop that does writeback and invalidates attached dentries. This can 
>>>> fail for various reasons.
>>> Yes, but I'm not at all sure that calling remove_inode_buffers() or
>>> invalidate_mapping_pages() is OK on a live inode.  They should be done
>>> after checking the refcount, just like prune_icache() does.
>> Dont we do the same on a truncate?
> 
> Yes, with i_mutex and i_alloc_sem held.

There is another call to invalidate_mapping_pages() in prune_icache (that is
where this code originates). No i_mutex and i_alloc. Only iprune_mutex held
and that seems to be for the protection of the list. So just checking
inode->i_count would do the trick?

>>> Also, while d_invalidate() is not actually wrong here, because you
>>> check S_ISDIR(), but it's still the wrong function to use.  You really
>>> just want to shrink the children.  Invalidation means: the filesystem
>>> found out that the cached inode is invalid, so we want to throw it
>>> away.  In the future it might actually be able to do it for
>>> directories as well, but currently it cannot because of possible
>>> mounts on the dentry.
>> Thats the same issue as with the dentries. The new function could deal with
>> both situations?
> 
> Sure.
> 
> The big issue is dealing with umount.  You could do something like
> grab_super() on sb before getting a ref on the inode/dentry.  But I'm
> not sure this is a good idea.  There must be a simpler way to achieve
> this...

Taking a lock on vfsmount_lock? But that would make dentry reclaim a pain.

We are only interested in the reclaim a dentry if its currently unused. If so
then why does unmount matter? Both unmount and reclaim will attempt to remove
the dentry.

Have a look at get_dentries(). It takes the dcache_lock and checks the dentry
state. Either the entry is ignored or dget_locked() removes it from the lru.
If its off the LRU then it can no longer be reclaimed by umount.


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