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

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Date:   Thu, 25 Mar 2021 18:15:11 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Roman Gushchin <guro@...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>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH 1/8] mm: cma: introduce cma_release_nowait()

On 25.03.21 17:56, Mike Kravetz wrote:
> On 3/25/21 3:22 AM, Michal Hocko wrote:
>> On Thu 25-03-21 10:56:38, David Hildenbrand wrote:
>>> 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?
>>
>> Good question. Looking at the code it doesn't seem that there is any
>> blockable operation or any heavy lifting done under the lock.
>> 7ee793a62fa8 ("cma: Remove potential deadlock situation") has introduced
>> the lock and there was a simple bitmat protection back then. I suspect
>> the patch just followed the cma_mutex lead and used the same type of the
>> lock. cma_mutex used to protect alloc_contig_range which is sleepable.
>>
>> This all suggests that there is no real reason to use a sleepable lock
>> at all and we do not need all this heavy lifting.
>>
> 
> When Roman first proposed these patches, I brought up the same issue:
> 
> https://lore.kernel.org/linux-mm/20201022023352.GC300658@carbon.dhcp.thefacebook.com/
> 
> Previously, Roman proposed replacing the mutex with a spinlock but
> Joonsoo was opposed.
> 
> Adding Joonsoo on Cc:
> 

There has to be a good reason not to. And if there is a good reason, 
lockless clearing might be one feasible alternative.

-- 
Thanks,

David / dhildenb