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Date: Thu, 16 Nov 2023 19:06:23 +0100
From: David Hildenbrand <david@...hat.com>
To: xu <xu.xin.sc@...il.com>
Cc: akpm@...ux-foundation.org, imbrenda@...ux.ibm.com,
jiang.xuexin@....com.cn, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, ran.xiaokai@....com.cn, wang.yong12@....com.cn,
xu.xin16@....com.cn, yang.yang29@....com.cn
Subject: Re: [PATCH] ksm: delay the check of splitting compound pages
On 16.11.23 18:39, David Hildenbrand wrote:
> 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]
>
Looking into the details, that doesn't work unfortunately.
try_to_merge_one_page() will call split_huge_page(), but that
will only hold a reference to "page", not to "tree page" after the
split.
Long story short, after split_huge_page() tree_page could get freed
because we don't hold a reference to that one anymore.
So we most probably cannot do better than the following (untested):
diff --git a/mm/ksm.c b/mm/ksm.c
index 7efcc68ccc6e..afb079524585 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2226,25 +2226,30 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
if (!err)
return;
}
+retry:
tree_rmap_item =
unstable_tree_search_insert(rmap_item, page, &tree_page);
if (tree_rmap_item) {
- bool split;
+ /*
+ * 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.
+ *
+ * We have to split manually, and lookup the tree_page
+ * again. Note that we'll only hold a reference to "page" after
+ * the split.
+ */
+ if (page_folio(page) == page_folio(tree_page)) {
+ put_page(tree_page);
+
+ if (!trylock_page(page))
+ return;
+ split_huge_page(page);
+ unlock_page(page);
+ goto retry;
+ }
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 (kpage) {
/*
@@ -2271,20 +2276,6 @@ 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);
}
}
}
--
Cheers,
David / dhildenb
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