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Date:   Wed, 11 Dec 2019 22:06:50 -0800
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     Waiman Long <longman@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v2] hugetlbfs: Disable softIRQ when taking hugetlb_lock
Message-ID: <20191212060650.ftqq27ftutxpc5hq@linux-p48b>
Mail-Followup-To: Mike Kravetz <mike.kravetz@oracle.com>,
        Waiman Long <longman@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Michal Hocko <mhocko@kernel.org>
References: <20191211194615.18502-1-longman@redhat.com>
 <4fbc39a9-2c9c-4c2c-2b13-a548afe6083c@oracle.com>
 <32d2d4f2-83b9-2e40-05e2-71cd07e01b80@redhat.com>
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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