linux-kernel - Re: [PATCH] radix_tree: take radix_tree

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Message-ID: <20111221221527.GE23662@dastard>
Date:	Thu, 22 Dec 2011 09:15:27 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] radix_tree: take radix_tree_path off stack

On Tue, Dec 20, 2011 at 10:53:17PM -0800, Hugh Dickins wrote:
> On Wed, 21 Dec 2011, Dave Chinner wrote:
> > On Sun, Dec 18, 2011 at 10:41:39PM -0800, Hugh Dickins wrote:
.....
> > >  		if (!(node = radix_tree_node_alloc(root)))
> > >  			return -ENOMEM;
> > >  
> > > -		/* Increase the height.  */
> > > -		node->slots[0] = indirect_to_ptr(root->rnode);
> > > -
> > >  		/* Propagate the aggregated tag info into the new root */
> > >  		for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
> > >  			if (root_tag_get(root, tag))
> > >  				tag_set(node, tag, 0);
> > >  		}
> > >  
> > > +		/* Increase the height.  */
> > >  		newheight = root->height+1;
> > 
> > While touching this code, fixing the adjacent whitespace damage
> > would be good.
> 
> I didn't notice any: do you mean "root->height+1" instead of
> "root->height + 1"?  I don't care much, and checkpatch didn't complain.

Yeah, that was what I was refering to.

> > >  		node->height = newheight;
> > >  		node->count = 1;
> > > +		node->parent = NULL;
> > > +		slot = root->rnode;
> > > +		if (newheight > 1) {
> > > +			slot = indirect_to_ptr(slot);
> > > +			slot->parent = node;
> > > +		}
> > > +		node->slots[0] = slot;
> > 
> > This would be much more obvious in function if it separated the two
> > different cases completely:
> > 
> > 		if (newheight > 1) {
> > 			slot = indirect_to_ptr(root->rnode);
> > 			slot->parent = node;
> > 		} else {
> > 			slot = root->rnode;
> > 			node->parent = NULL;
> > 		}
> > 		node->slots[0] = slot;
> 
> We do need to set node->parent NULL in all cases (and cannot clear
> it when freeing).  I chose the "slot = blah(slot)" style to follow the
> "newptr = blah(newptr)" over in radix_tree_shrink(), thought it helped
> to keep those blocks alike.

You're right. I really was being dense yesterday. To tell the truth,
though, I found the "newptr" style easier to follow because it was
obvious which was the object being initialised. I think that it not
being obvious which object needed full initialisation contribted to
my mix up of node and slot parent pointers in my above comment...

> > > @@ -701,15 +691,21 @@ unsigned long radix_tree_range_tag_if_ta
> > >  		tag_set(slot, settag, offset);
> > >  
> > >  		/* walk back up the path tagging interior nodes */
> > > -		pathp = &path[0];
> > > -		while (pathp->node) {
> > > +		upindex = index;
> > > +		while (node) {
> > > +			upindex >>= RADIX_TREE_MAP_SHIFT;
> > > +			offset = upindex & RADIX_TREE_MAP_MASK;
> > > +
> > >  			/* stop if we find a node with the tag already set */
> > > -			if (tag_get(pathp->node, settag, pathp->offset))
> > > +			if (tag_get(node, settag, offset))
> > >  				break;
> > > -			tag_set(pathp->node, settag, pathp->offset);
> > > -			pathp++;
> > > +			tag_set(node, settag, offset);
> > > +			node = node->parent;
> > >  		}
> > >  
> > > +		/* optimization: no need to walk up from this node again */
> > > +		node = NULL;
> > 
> > As per my query above: why? That's the question the comment needs to
> > answer....
> 
> At the top of the hunk, we can see the tag_set(slot, settag, offset)
> where it sets the tag in the leafnode "slot"; then it loops up to parent
> "node" of slot, to parent of parent, etc, setting tag in those, but
> breaking as soon as it finds the tag already set - it can be sure that
> the tag must already be set on all nodes above.
> 
> If afterwards it comes to set tag at another offset (most likely the
> very next) in this same leafnode, we know that it has already set tag
> on the parent, the parent's parent etc., so need not bother to tag_get
> from the level above to discover that.  And since we happen to have a
> variable "node" which stops the loop when it's NULL, let's set it to
> NULL now to stop the loop immediately in future.

Ok, gotcha. perhaps a more expansive comment along the lines of:

/*
 * we can clear the node pointer now as all it's ancestors have the
 * tage set due to setting it on the slot above. Hence we have no
 * need to walk back up the tree to set tags if there is no further
 * tags to set.
 */

is in order to remind me in a few months time why it this was done?

Cheers,

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