Re: SLUB defrag pull request?

From: Christoph Lameter
Date: Mon Oct 20 2008 - 14:24:04 EST

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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