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SubjectRe: [PATCH 1/8] mm: cma: introduce cma_release_nowait()
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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@fb.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@fb.com>
>>>> [mike.kravetz@oracle.com: rebased to v5.12-rc3-mmotm-2021-03-17-22-24]
>>>> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.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

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