>>>> @@ -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?
--- 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);
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.
- */
- split = PageTransCompound(page)
- && compound_head(page) == compound_head(tree_page);
- put_page(tree_page);
+
+ if (!SameCompound)
+ put_page(tree_page);
+
if (kpage) {
/*
* The pages were successfully merged: insert new
@@ -2271,20 +2269,6 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
break_cow(tree_rmap_item);
break_cow(rmap_item);
}
- } else if (split) {
- /*
- * We are here if we tried to merge two pages and
- * failed because they both belonged to the same
- * compound page. We will split the page now, but no
- * merging will take place.
- * We do not want to add the cost of a full lock; if
- * the page is locked, it is better to skip it and
- * perhaps try again later.
- */
- if (!trylock_page(page))
- return;
- split_huge_page(page);
- unlock_page(page);
}
}
}
>>
>> I don't know the original reason, better ask Claudio Imbrenda <imbrenda@xxxxxxxxxxxxxxxxxx>.
>> Maybe because doing detection that ahead of time will break several funtions' semantic,
>> such as try_to_merge_two_pages(), try_to_merge_with_ksm_page() and try_to_merge_one_page()
>>
>> Adding the backup code don't change the old code and fixing the old problem, it's good.
>
>It's absolutely counter-intuitive to check for something that cannot
>possibly work after the effects. This better has a good reason to make
>that code more complicated.
>--
