Re: [RFC PATCH 3/3] mm: increase scalability of global memory commitment accounting

[Andrey Ryabinin]

On 02/11/2016 11:51 PM, Andrew Morton wrote:
> On Wed, 10 Feb 2016 16:24:16 -0800 Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:
> 
>> On Wed, 2016-02-10 at 13:28 -0800, Andrew Morton wrote:
>>
>>>
>>> If a process is unmapping 4MB then it's pretty crazy for us to be
>>> hitting the percpu_counter 32 separate times for that single operation.
>>>
>>> Is there some way in which we can batch up the modifications within the
>>> caller and update the counter less frequently?  Perhaps even in a
>>> single hit?
>>
>> I think the problem is the batch size is too small and we overflow
>> the local counter into the global counter for 4M allocations.
> 
> That's one way of looking at the issue.  The other way (which I point
> out above) is that we're calling vm_[un]_acct_memory too frequently
> when mapping/unmapping 4M segments.
> 

We call it only once per mmap() or munmap(), so there is nothing to improve.

> Exactly which mmap.c callsite is causing this issue?
> 


mmap_region() (or do_brk()) ->
	security_vm_enough_memory() ->
		__vm_enough_memory() ->
			vm_acct_memory()