On Tue, 17 Dec 2019, 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. However, Mike Kravetz had learned that the hugetlb_lock is
held for a linear scan of ALL hugetlb pages during a cgroup reparentling
operation. So it is just too long to have irq disabled unless we can
break hugetbl_lock down into finer-grained locks with shorter lock
hold times.
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 using the llist APIs.
The generic workqueue is used to process the work, but a dedicated
workqueue can be used instead if it is desirable to have the huge page
freed ASAP.
Thanks to Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> for suggesting the use
of llist APIs which simplfy the code.
[v2: Add more comment & remove unneeded racing check]
[v3: Update commit log, remove pr_debug & use llist APIs]
Very creative reusing the mapping pointer, along with the llist api,
this solves the problem nicely (temporarily at least).
Two small nits below.
Acked-by: Davidlohr Bueso <dbueso@xxxxxxx>
Reported-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
mm/hugetlb.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 50 insertions(+), 1 deletion(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
+static LLIST_HEAD(hpage_freelist);
+
+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((struct address_space **)node,
+ struct page, mapping);
+ node = node->next;
llist_next()
+ __free_huge_page(page);
+ }
+}
+static DECLARE_WORK(free_hpage_work, free_hpage_workfn);
+
+void free_huge_page(struct page *page)
+{
+ /*
+ * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
+ */
+ if (!in_task()) {
unlikely()?
Thanks,
Davidlohr
