linux-kernel - Re: [PATCH 1/8] mm: cma: introduce cma_release

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Date:   Thu, 25 Mar 2021 10:56:38 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Oscar Salvador <osalvador@...e.de>,
        Muchun Song <songmuchun@...edance.com>,
        David Rientjes <rientjes@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        Hillf Danton <hdanton@...a.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/8] mm: cma: introduce cma_release_nowait()

On 25.03.21 01:28, Mike Kravetz wrote:
> From: Roman Gushchin <guro@...com>
> 
> cma_release() has to lock the cma_lock mutex to clear the cma bitmap.
> It makes it a blocking function, which complicates its usage from
> non-blocking contexts. For instance, hugetlbfs code is temporarily
> dropping the hugetlb_lock spinlock to call cma_release().
> 
> This patch introduces a non-blocking cma_release_nowait(), which
> postpones the cma bitmap clearance. It's done later from a work
> context. The first page in the cma allocation is used to store
> the work struct. Because CMA allocations and de-allocations are
> usually not that frequent, a single global workqueue is used.
> 
> To make sure that subsequent cma_alloc() call will pass, cma_alloc()
> flushes the cma_release_wq workqueue. To avoid a performance
> regression in the case when only cma_release() is used, gate it
> by a per-cma area flag, which is set by the first call
> of cma_release_nowait().
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> [mike.kravetz@...cle.com: rebased to v5.12-rc3-mmotm-2021-03-17-22-24]
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---


1. Is there a real reason this is a mutex and not a spin lock? It seems 
to only protect the bitmap. Are bitmaps that huge that we spend a 
significant amount of time in there?

Because I also read "Because CMA allocations and de-allocations are
usually not that frequent".

With a spinlock, you would no longer be sleeping, but obviously you 
might end up waiting for the lock ;) Not sure if that would help.

2. IIUC, if we would do the clearing completely lockless and use atomic 
bitmap ops instead, only cma_debug_show_areas() would see slight 
inconsistencies. As long as the setting code (-> allocation code) holds 
the lock, I think this should be fine (-> no double allocations).

(sorry if that has already been discussed)

-- 
Thanks,

David / dhildenb