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SubjectRe: [PATCH v2] hugetlbfs: Disable softIRQ when taking hugetlb_lock
On Wed, 11 Dec 2019, Mike Kravetz wrote:
>The workqueue approach would address both soft and hard irq context issues.
>As a result, I too think this is the approach we should explore. Since there
>is more than one lock involved, this also is reason for a work queue approach.
>
>I'll take a look at initial workqueue implementation. However, I have not
>dealt with workqueues in some time so it may take few days to evaluate.

I'm thinking of something like the following; it at least passes all ltp
hugetlb related testcases.

Thanks,
Davidlohr

----8<------------------------------------------------------------------
[PATCH] mm/hugetlb: defer free_huge_page() to a workqueue

There have been deadlock reports[1, 2] where put_page is called
from softirq context and this causes trouble with the hugetlb_lock,
as well as potentially the subpool lock.

For such an unlikely scenario, lets not add irq dancing overhead
to the lock+unlock operations, which could incur in expensive
instruction dependencies, particularly when considering hard-irq
safety. For example PUSHF+POPF on x86.

Instead, just use a workqueue and do the free_huge_page() in regular
task context.

[1] https://lore.kernel.org/lkml/20191211194615.18502-1-longman@redhat.com/
[2] https://lore.kernel.org/lkml/20180905112341.21355-1-aneesh.kumar@linux.ibm.com/

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
mm/hugetlb.c | 38 +++++++++++++++++++++++++++++++++++++-
1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ac65bb5e38ac..737108d8d637 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1136,8 +1136,17 @@ static inline void ClearPageHugeTemporary(struct page *page)
page[2].mapping = NULL;
}

-void free_huge_page(struct page *page)
+static struct workqueue_struct *hugetlb_free_page_wq;
+struct hugetlb_free_page_work {
+ struct page *page;
+ struct work_struct work;
+};
+
+static void free_huge_page_workfn(struct work_struct *work)
{
+ struct page *page = container_of(work,
+ struct hugetlb_free_page_work,
+ work)->page;
/*
* Can't pass hstate in here because it is called from the
* compound page destructor.
@@ -1197,6 +1206,27 @@ void free_huge_page(struct page *page)
enqueue_huge_page(h, page);
}
spin_unlock(&hugetlb_lock);
+
+}
+
+/*
+ * While unlikely, free_huge_page() can be at least called from
+ * softirq context, defer freeing such that the hugetlb_lock and
+ * spool->lock need not have to deal with irq dances just for this.
+ */
+void free_huge_page(struct page *page)
+{
+ struct hugetlb_free_page_work work;
+
+ work.page = page;
+ INIT_WORK_ONSTACK(&work.work, free_huge_page_workfn);
+ queue_work(hugetlb_free_page_wq, &work.work);
+
+ /*
+ * Wait until free_huge_page is done.
+ */
+ flush_work(&work.work);
+ destroy_work_on_stack(&work.work);
}

static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
@@ -2816,6 +2846,12 @@ static int __init hugetlb_init(void)

for (i = 0; i < num_fault_mutexes; i++)
mutex_init(&hugetlb_fault_mutex_table[i]);
+
+ hugetlb_free_page_wq = alloc_workqueue("hugetlb_free_page_wq",
+ WQ_MEM_RECLAIM, 0);
+ if (!hugetlb_free_page_wq)
+ return -ENOMEM;
+
return 0;
}
subsys_initcall(hugetlb_init);
--
2.16.4
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