On Thu, Aug 25, 2011 at 12:03:47PM +0530, Prateek Sharma wrote:
> Hello everyone .
> I've been trying to understand how KSM works (i want to make some
> modifications / implement some optimizations) .
> One thing that struck me odd was the high number of calls to
> remove_rmap_item_from_tree .
> Particularly, this instance in cmp_and_merge_page :
>
> /*
> * As soon as we merge this page, we want to remove the
> * rmap_item of the page we have merged with from the unstable
> * tree, and insert it instead as new node in the stable tree.
> */
> if (kpage) {
> remove_rmap_item_from_tree(tree_rmap_item);
>
> lock_page(kpage);
> stable_node = stable_tree_insert(kpage);
> if (stable_node) {
> stable_tree_append(tree_rmap_item, stable_node);
> stable_tree_append(rmap_item, stable_node);
> }
>
> Here, from i understand, we've found a match in the unstable tree, and
> are adding a stable node in the stable tree.
> My question is: why do we need to remove the rmap_item from unstable
> tree here? At the end of a scan we are erasing the unstable tree
> anyway.
It does not make sense for the item to be in the unstable tree anymore:
it now represents two write protected pages, so should reside in the
stable tree.
> Also, all searches first consider the stable tree , and then
> the unstable tree.
> What will happen if we find a match in the unstable tree, and simply
> update tree_rmap_item to point to a stable_node ?
I suppose its simpler to remove from unstable and add to stable tree.
>
> Thanks for reading. I'd love to share the ideas i have for (attempting
> to) improve KSM, if anyone is interested.
>
> Prateek
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