linux-kernel - Re: [PATCH v2] mm/hugetlb: defer free_huge

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Date:   Thu, 12 Dec 2019 15:52:20 -0500
From:   Waiman Long <longman@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...nel.org>, aneesh.kumar@...ux.ibm.com
Subject: Re: [PATCH v2] mm/hugetlb: defer free_huge_page() to a workqueue

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@...hat.com>
>> Reported-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
>> Signed-off-by: Davidlohr Bueso <dbueso@...e.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.

-Longman