Re: [PATCH v4 72/73] xfs: Convert mru cache to XArray

[Matthew Wilcox <willy@infradead.org>]

On Wed, Dec 06, 2017 at 12:36:48PM +1100, Dave Chinner wrote:
> > -	if (radix_tree_preload(GFP_NOFS))
> > -		return -ENOMEM;
> > -
> >  	INIT_LIST_HEAD(&elem->list_node);
> >  	elem->key = key;
> >  
> > -	spin_lock(&mru->lock);
> > -	error = radix_tree_insert(&mru->store, key, elem);
> > -	radix_tree_preload_end();
> > -	if (!error)
> > -		_xfs_mru_cache_list_insert(mru, elem);
> > -	spin_unlock(&mru->lock);
> > +	do {
> > +		xas_lock(&xas);
> > +		xas_store(&xas, elem);
> > +		error = xas_error(&xas);
> > +		if (!error)
> > +			_xfs_mru_cache_list_insert(mru, elem);
> > +		xas_unlock(&xas);
> > +	} while (xas_nomem(&xas, GFP_NOFS));
> 
> Ok, so why does this have a retry loop on ENOMEM despite the
> existing code handling that error? And why put such a loop in this
> code and not any of the other XFS code that used
> radix_tree_preload() and is arguably much more important to avoid
> ENOMEM on insert (e.g. the inode cache)?

If we need more nodes in the tree, xas_store() will try to allocate them
with GFP_NOWAIT | __GFP_NOWARN.  If that fails, it signals it in xas_error().
xas_nomem() will notice that we're in an ENOMEM situation, and allocate
a node using your preferred GFP flags (NOIO in your case).  Then we retry,
guaranteeing forward progress. [1]

The other conversions use the normal API instead of the advanced API, so
all of this gets hidden away.  For example, the inode cache does this:

+       curr = xa_cmpxchg(&pag->pag_ici_xa, agino, NULL, ip, GFP_NOFS);

and xa_cmpxchg internally does:

        do {
                xa_lock_irqsave(xa, flags);
                curr = xas_create(&xas);
                if (curr == old)
                        xas_store(&xas, entry);
                xa_unlock_irqrestore(xa, flags);
        } while (xas_nomem(&xas, gfp));


> Also, I really don't like the pattern of using xa_lock()/xa_unlock()
> to protect access to an external structure. i.e. the mru->lock
> context is protecting multiple fields and operations in the MRU
> structure, not just the radix tree operations. Turning that around
> so that a larger XFS structure and algorithm is now protected by an
> opaque internal lock from generic storage structure the forms part
> of the larger structure seems like a bad design pattern to me...

It's the design pattern I've always intended to use.  Naturally, the
xfs radix trees weren't my initial target; it was the page cache, and
the page cache does the same thing; uses the tree_lock to protect both
the radix tree and several other fields in that same data structure.

I'm open to argument on this though ... particularly if you have a better
design pattern in mind!

[1] I actually have this documented!  It's in the xas_nomem() kernel-doc:

 * If we need to add new nodes to the XArray, we try to allocate memory
 * with GFP_NOWAIT while holding the lock, which will usually succeed.
 * If it fails, @xas is flagged as needing memory to continue.  The caller
 * should drop the lock and call xas_nomem().  If xas_nomem() succeeds,
 * the caller should retry the operation.
 *
 * Forward progress is guaranteed as one node is allocated here and
 * stored in the xa_state where it will be found by xas_alloc().  More
 * nodes will likely be found in the slab allocator, but we do not tie
 * them up here.
 *
 * Return: true if memory was needed, and was successfully allocated.