Re: [PATCH 1/8] mm: cma: introduce cma_release_nowait()

[Mike Kravetz]

On 3/25/21 4:19 PM, Roman Gushchin wrote:
> On Thu, Mar 25, 2021 at 01:12:51PM -0700, Minchan Kim wrote:
>> On Thu, Mar 25, 2021 at 06:15:11PM +0100, David Hildenbrand wrote:
>>> 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@xxxxxx>
>>>>>>>
>>>>>>> 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@xxxxxx>
>>>>>>> [mike.kravetz@xxxxxxxxxx: rebased to v5.12-rc3-mmotm-2021-03-17-22-24]
>>>>>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>>>>>> ---
>>>>>>
>>>>>>
>>>>>> 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@xxxxxxxxxxxxxxxxxxxxxxxxxxx/
>>>>
>>>> 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.
>>
>> I also don't think nowait variant is good idea. If the scanning of
>> bitmap is *really* significant, it might be signal that we need to
>> introduce different technique or data structure not bitmap rather
>> than a new API variant.
> 
> I'd also prefer to just replace the mutex with a spinlock rather than fiddling
> with a delayed release.
> 

I hope Joonsoo or someone else brings up specific concerns.  I do not
know enough about all CMA use cases.  Certainly, in this specific use
case converting to a spinlock would not be an issue.  Do note that we
would want to convert to an irq safe spinlock and disable irqs if that
makes any difference in the discussion.
-- 
Mike Kravetz