linux-kernel - Re: vmevent: question?

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Date:	Mon, 30 Apr 2012 17:36:26 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Pekka Enberg <penberg@...nel.org>
CC:	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Anton Vorontsov <anton.vorontsov@...aro.org>,
	Leonid Moiseichuk <leonid.moiseichuk@...ia.com>
Subject: Re: vmevent: question?

On 04/30/2012 05:01 PM, Pekka Enberg wrote:

> Hi Minchan,
> 
> On Mon, Apr 30, 2012 at 10:52 AM, Minchan Kim <minchan@...nel.org> wrote:
>>> It makes the userspace side simpler for "lowmem notification" use
>>> case. I'm open to changing the ABI if it doesn't make the userspace
>>> side too complex.
>>
>> Yes. I understand your point but if we still consider all of values,
>> we don't have any way to capture exact values except triggered event value.
>> I mean there is no lock to keep consistency.
>> If stale data is okay, no problem but IMHO, it could make user very confusing.
>> So let's return value for first matched event if various event match.
>> Of course, let's write down it in ABI.
>> If there is other idea for reporting all of item with consistent, I'm okay.
> 
> What kind of consistency guarantees do you mean? The data sent to
> userspace is always a snapshot of the state and therefore can be stale
> by the time it reaches userspace.


Consistency between component of snapshot.
let's assume following as

1. User expect some events's value would be minus when event he expect happen.
   A : -3, B : -4, C : -5, D : -6
2. Logically, it's not possible to mix plus and minus values for the events.
   A : -3, B : -4, C : -5, D : -6 ( O )
   A : -3, B : -4, C : 1, D : 2   ( X )
   
But in current implementation, some of those could be minus and some of those could be plus.
Which event could user believe?
At least, we need a _captured_ value when event triggered so that user can ignore other values.

> 
> If your code needs stricter consistency guarantees, you probably want
> to do it in the kernel.
> 
>                                 Pekka
> 
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-- 
Kind regards,
Minchan Kim
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