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Subject: Re: [PATCH v2] mm/hugetlb: defer free_huge_page() to a workqueue
From:   Waiman Long <longman@redhat.com>
To:     Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Michal Hocko <mhocko@kernel.org>, aneesh.kumar@linux.ibm.com
References: <20191211194615.18502-1-longman@redhat.com>
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Organization: Red Hat
Message-ID: <3bdddd47-c93e-6549-d36f-8eb8e8d9be12@redhat.com>
Date:   Thu, 12 Dec 2019 16:04:29 -0500
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On 12/12/19 3:52 PM, Waiman Long wrote:
> On 12/12/19 2:22 PM, Mike Kravetz wrote:
>> On 12/12/19 11:04 AM, Davidlohr Bueso wrote:
>>> 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/
>>>
>>> Reported-by: Waiman Long <longman@redhat.com>
>>> Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>>> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
>> Thank you Davidlohr.
>>
>> The patch does seem fairly simple and straight forward.  I need to brush up
>> on my workqueue knowledge to provide a full review.
>>
>> Longman,
>> Do you have a test to reproduce the issue?  If so, can you try running with
>> this patch.
> Yes, I do have a test that can reproduce the issue. I will run it with
> the patch and report the status tomorrow.

I don't think Davidlohr's patch is ready for prime time yet. So I will
wait until a better version is available.

Cheers,
Longman