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

[Mike Kravetz]

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