linux-kernel - Re: [RFC PATCH 7/8] hugetlb: add update_and_free_page_no

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Date:   Mon, 22 Mar 2021 09:41:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Rientjes <rientjes@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        HORIGUCHI NAOYA <naoya.horiguchi@....com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Waiman Long <longman@...hat.com>, Peter Xu <peterx@...hat.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 7/8] hugetlb: add update_and_free_page_no_sleep for
 irq context

On Fri, Mar 19, 2021 at 03:42:08PM -0700, Mike Kravetz wrote:
> The locks acquired in free_huge_page are irq safe.  However, in certain
> circumstances the routine update_and_free_page could sleep.  Since
> free_huge_page can be called from any context, it can not sleep.
> 
> Use a waitqueue to defer freeing of pages if the operation may sleep.  A
> new routine update_and_free_page_no_sleep provides this functionality
> and is only called from free_huge_page.
> 
> Note that any 'pages' sent to the workqueue for deferred freeing have
> already been removed from the hugetlb subsystem.  What is actually
> deferred is returning those base pages to the low level allocator.

So maybe I'm stupid, but why do you need that work in hugetlb? Afaict it
should be in cma_release().

Also, afaict cma_release() does free_contig_range() *first*, and then
does the 'difficult' bits. So how about you re-order
free_gigantic_page() a bit to make it unconditionally do
free_contig_range() and *then* call into CMA, which can then do a
workqueue thingy if it feels like it.

That way none of the hugetlb accounting is delayed, and only CMA gets to
suffer.