KSM Unstable tree questiom

KSM Unstable tree questiom

[Prateek Sharma]

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. 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 ?

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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