Re: [PATCH] ksm: delay the check of splitting compound pages

From: David Hildenbrand
Date: Thu Nov 16 2023 - 12:39:12 EST


On 16.11.23 13:17, xu wrote:
@@ -2229,24 +2229,10 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
tree_rmap_item =
unstable_tree_search_insert(rmap_item, page, &tree_page);
if (tree_rmap_item) {
- bool split;
-
kpage = try_to_merge_two_pages(rmap_item, page,
tree_rmap_item, tree_page);
- /*
- * If both pages we tried to merge belong to the same compound
- * page, then we actually ended up increasing the reference
- * count of the same compound page twice, and split_huge_page
- * failed.
- * Here we set a flag if that happened, and we use it later to
- * try split_huge_page again. Since we call put_page right
- * afterwards, the reference count will be correct and
- * split_huge_page should succeed.
- */

I'm curious, why can't we detect that ahead of time and keep only a
single reference? Why do we need the backup code? Anything I am missing?

Do you mean like this?

Let me see if the refcounting here is sane:

(a) The caller has a reference on "page" that it will put just after the
cmp_and_merge_page() call.
(b) unstable_tree_search_insert() obtained a reference to the
"tree_page" using get_mergeable_page()->follow_page(). We will put
that reference.

So indeed, if both references are to the same folio, *we* have two references to the folio and can safely drop one of both.


--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2229,23 +2229,21 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
tree_rmap_item =
unstable_tree_search_insert(rmap_item, page, &tree_page);
if (tree_rmap_item) {
- bool split;
+ bool SameCompound;
+ /*
+ * If they belongs to the same compound page, its' reference
+ * get twice, so need to put_page once to avoid that
+ * split_huge_page fails in try_to_merge_two_pages().
+ */
+ if (SameCompound = Is_SameCompound(page, tree_page))
+ put_page(tree_page);

bool same_folio = page_folio(page) == page_folio(tree_page);

/*
* If both pages belong to the same folio, we are holding two references
* to the same large folio: splitting the folio in
* try_to_merge_one_page() will fail for that reason. So let's just drop
* one reference early. Note that this is only possible if tree_page is
* not a KSM page yet.
*/
if (same_folio)
put_page(tree_page);

[we could use more folio operations here, but lets KIS]

--
Cheers,

David / dhildenb