linux-kernel - Re: [PATCH v2 1/8] mm/cma: change cma mutex to irq safe spinlock

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Date:   Tue, 30 Mar 2021 19:37:39 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     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>,
        David Hildenbrand <david@...hat.com>,
        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>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 1/8] mm/cma: change cma mutex to irq safe spinlock

On 3/30/21 1:01 AM, Michal Hocko wrote:
> On Mon 29-03-21 16:23:55, Mike Kravetz wrote:
>> Ideally, cma_release could be called from any context.  However, that is
>> not possible because a mutex is used to protect the per-area bitmap.
>> Change the bitmap to an irq safe spinlock.
> 
> I would phrase the changelog slightly differerent
> "
> cma_release is currently a sleepable operatation because the bitmap
> manipulation is protected by cma->lock mutex. Hugetlb code which relies
> on cma_release for CMA backed (giga) hugetlb pages, however, needs to be
> irq safe.
> 
> The lock doesn't protect any sleepable operation so it can be changed to
> a (irq aware) spin lock. The bitmap processing should be quite fast in
> typical case but if cma sizes grow to TB then we will likely need to
> replace the lock by a more optimized bitmap implementation.
> "

That is better.  Thank you.

> 
> it seems that you are overusing irqsave variants even from context which
> are never called from the IRQ context so they do not need storing flags.
> 
> [...]

Yes.

>> @@ -391,8 +391,9 @@ static void cma_debug_show_areas(struct cma *cma)
>>  	unsigned long start = 0;
>>  	unsigned long nr_part, nr_total = 0;
>>  	unsigned long nbits = cma_bitmap_maxno(cma);
>> +	unsigned long flags;
>>  
>> -	mutex_lock(&cma->lock);
>> +	spin_lock_irqsave(&cma->lock, flags);
> 
> spin_lock_irq should be sufficient. This is only called from the
> allocation context and that is never called from IRQ context.
> 

I will change this and those below.

Thanks for your continued reviews and patience.
-- 
Mike Kravetz