Re: [PATCH v2] mm/hugetlb: Defer freeing of huge pages if in non-task context

[Kirill Tkhai]

On 17.12.2019 17:00, Waiman Long wrote:
> On 12/17/19 5:50 AM, Kirill Tkhai wrote:
>> On 17.12.2019 12:31, Michal Hocko wrote:
>>> On Mon 16-12-19 20:25:08, Waiman Long wrote:
>>> [...]
>>>> Both the hugetbl_lock and the subpool lock can be acquired in
>>>> free_huge_page(). One way to solve the problem is to make both locks
>>>> irq-safe.
>>> Please document why we do not take this, quite natural path and instead
>>> we have to come up with an elaborate way instead. I believe the primary
>>> motivation is that some operations under those locks are quite
>>> expensive. Please add that to the changelog and ideally to the code as
>>> well. We probably want to fix those anyway and then this would be a
>>> temporary workaround.
>>>
>>>> Another alternative is to defer the freeing to a workqueue job.
>>>>
>>>> This patch implements the deferred freeing by adding a
>>>> free_hpage_workfn() work function to do the actual freeing. The
>>>> free_huge_page() call in a non-task context saves the page to be freed
>>>> in the hpage_freelist linked list in a lockless manner.
>>> Do we need to over complicate this (presumably) rare event by a lockless
>>> algorithm? Why cannot we use a dedicated spin lock for for the linked
>>> list manipulation? This should be really a trivial code without an
>>> additional burden of all the lockless subtleties.
>> Why not llist_add()/llist_del_all() ?
>>
> The llist_add() and llist_del_all() are just simple helpers. Because
> this lockless case involve synchronization of two variables, the llist
> helpers do not directly apply here. So the rests cannot be used. It will
> look awkward it is partially converted to use the helpers. If we convert
> to use a lock as suggested by Michal, using the helpers will be an
> overkill as xchg() will not be needed.

I don't understand you. What are two variables?

Why can't you simply do the below?

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ac65bb5e38ac..e8ec753f3d92 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1136,7 +1136,7 @@ static inline void ClearPageHugeTemporary(struct page *page)
 	page[2].mapping = NULL;
 }
 
-void free_huge_page(struct page *page)
+static void __free_huge_page(struct page *page)
 {
 	/*
 	 * Can't pass hstate in here because it is called from the
@@ -1199,6 +1199,35 @@ void free_huge_page(struct page *page)
 	spin_unlock(&hugetlb_lock);
 }
 
+static struct llist_head hpage_freelist = LLIST_HEAD_INIT;
+
+static void free_hpage_workfn(struct work_struct *work)
+{
+	struct llist_node *node;
+	struct page *page;
+
+	node = llist_del_all(&hpage_freelist);
+
+	while (node) {
+		page = container_of(node, struct page, mapping);
+		node = node->next;
+		__free_huge_page(page);
+	}
+}
+
+static DECLARE_WORK(free_hpage_work, free_hpage_workfn);
+
+void free_huge_page(struct page *page)
+{
+	if (!in_task()) {
+		if (llist_add((struct llist_node *)&page->mapping, &hpage_freelist))
+			schedule_work(&free_hpage_work);
+		return;
+	}
+
+	__free_huge_page(page);
+}
+
 static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
 {
 	INIT_LIST_HEAD(&page->lru);