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

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Message-ID: <20191216171838.GA32169@bombadil.infradead.org>
Date:   Mon, 16 Dec 2019 09:18:38 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Waiman Long <longman@...hat.com>, 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 Mon, Dec 16, 2019 at 08:17:48AM -0800, Davidlohr Bueso wrote:
> On Mon, 16 Dec 2019, Michal Hocko wrote:
> > I am afraid that work_struct is too large to be stuffed into the struct
> > page array (because of the lockdep part).
> 
> Yeah, this needs to be done without touching struct page.
> 
> Which is why I had done the stack allocated way in this patch, but we
> cannot wait for it to complete in irq, so that's out the window. Andi
> had suggested percpu allocated work items, but having played with the
> idea over the weekend, I don't see how we can prevent another page being
> freed on the same cpu before previous work on the same cpu is complete
> (cpu0 wants to free pageA, schedules the work, in the mean time cpu0
> wants to free pageB and workerfn for pageA still hasn't been called).

Why is it that we can call functions after-an-RCU-period-has-elapsed time,
at returning-to-userspace time and after-exiting-hardirq-handler
time easily, but the mechanism for calling a function
after-we've-finished-handling-softirqs is so bloody hard to use?

That's surely the major problem we need to fix.