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Date:   Mon, 22 Mar 2021 09:41:01 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Michal Hocko <mhocko@suse.com>,
        Shakeel Butt <shakeelb@google.com>,
        Oscar Salvador <osalvador@suse.de>,
        David Hildenbrand <david@redhat.com>,
        Muchun Song <songmuchun@bytedance.com>,
        David Rientjes <rientjes@google.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Matthew Wilcox <willy@infradead.org>,
        HORIGUCHI NAOYA <naoya.horiguchi@nec.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
        Waiman Long <longman@redhat.com>, Peter Xu <peterx@redhat.com>,
        Mina Almasry <almasrymina@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 7/8] hugetlb: add update_and_free_page_no_sleep for
 irq context
Message-ID: <YFhYHZ9onwdZMeDi@hirez.programming.kicks-ass.net>
References: <20210319224209.150047-1-mike.kravetz@oracle.com>
 <20210319224209.150047-8-mike.kravetz@oracle.com>
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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.