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

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Date:   Mon, 16 Dec 2019 14:37:11 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Waiman Long <longman@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        aneesh.kumar@...ux.ibm.com
Subject: Re: [PATCH v2] mm/hugetlb: defer free_huge_page() to a workqueue

On Thu 12-12-19 11:04:27, 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.

I am afraid that work_struct is too large to be stuffed into the struct
page array (because of the lockdep part).

I think that it would be just safer to make hugetlb_lock irq safe. Are
there any other locks that would require the same?
-- 
Michal Hocko
SUSE Labs