Re: [PATCH v2 18/29] memcg: kmem controller charge/uncharge infrastructure

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2012/05/16 17:25), Glauber Costa wrote:

> On 05/16/2012 12:18 PM, KAMEZAWA Hiroyuki wrote:
>>> If at this point the memcg hits a NOFAIL allocation worth 2 pages, by
>>>>  the method I am using, the memcg will be at 4M + 4k after the
>>>>  allocation. Charging it to the root memcg will leave it at 4M - 4k.
>>>>  
>>>>  This means that to be able to allocate a page again, you need to free
>>>>  two other pages, be it the 2 pages used by the GFP allocation or any
>>>>  other. In other words: the memcg that originated the charge is held
>>>>  accountable for it. If he says it can't fail for whatever reason, fine,
>>>>  we respect that,  but we punish it later for other allocations.
>>>>  
>> I personally think 'we punish it later' is bad thing at resource accounting.
>> We have 'hard limit'. It's not soft limit.
> 
> That only makes sense if you will fail the allocation. If you won't, you
> are over your hard limit anyway. You are just masquerading that.
> 


'showing usage > limit to user' and 'avoid accounting'
is totally different user experience.



>>>>  Without that GFP_NOFAIL becomes just a nice way for people to bypass
>>>>  those controls altogether, since after a ton of GFP_NOFAIL allocations,
>>>>  normal allocations will still succeed.
>>>>  
>> Allowing people to bypass is not bad because they're kernel.
> 
> No, they are not. They are in process context, on behalf of a process
> that belongs to a valid memcg. If they happen to be a kernel thread,
> !current->mm test will send the allocation to the root memcg already.
> 


Yes, but it's kernel code. There will be some special reason to use __GFP_NOFAIL.

>>
>> But, IIUC, from gfp.h
>> ==
>>   * __GFP_NOFAIL: The VM implementation_must_  retry infinitely: the caller
>>   * cannot handle allocation failures.  This modifier is deprecated and no new
>>   * users should be added.
>> ==
>>
>> GFP_NOFAIL will go away and no new user is recommended.
>>
> Yes, I am aware of that. That's actually why I don't plan to insist on
> this too much - although your e-mail didn't really convince me.
> 
> It should not matter in practice.
> 
>> So, please skip GFP_NOFAIL accounting and avoid to write
>> "usage may go over limit if you're unfortune, sorry" into memcg documentation.
> 
> I won't write that, because that's not true. Is more like: "Allocations
> that can fail will fail if you go over limit".
> 

>>
>>>>  The change you propose is totally doable. I just don't believe it should
>>>>  be done.
>>>>  
>>>>  But let me know where you stand.
>>>>  
>> My stand point is keeping "usage<= limit" is the spec. and
>> important in enterprise system. So, please avoid usage>  limit.
>>
> As I said, I won't make a case here because those allocations shouldn't
> matter in real life anyway. I can change it.
> 

My standing point is that 'usage > limit' is bug. So please avoid it if
__GFP_NOFAIL allocation is not very important.


Thanks,
-Kame


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