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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