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

[Mike Kravetz <mike.kravetz@oracle.com>]

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:
-- 
Mike Kravetz