Re: Allow migration of mlocked page?

[Minchan Kim <minchan@kernel.org>]

On 05/11/2012 06:20 PM, Peter Zijlstra wrote:

> On Fri, 2012-05-11 at 13:37 +0900, Minchan Kim wrote:
>> I hope hear opinion from rt guys, too.
> 
> Its a problem yes, not sure your solution is any good though. As it
> stands mlock() simply doesn't guarantee no faults, all it does is
> guarantee no major faults.


I can't find such definition from man pages
"
       Real-time  processes  that are using mlockall() to prevent delays on page faults should
       reserve enough locked stack pages before entering the time-critical section, so that no
       page fault can be caused by function calls
"
So I didn't expect it. Is your definition popular available on server RT?
At least, embedded guys didn't expect it.


> 
> Are you saying compaction doesn't actually move mlocked pages? I'm


Yes.

> somewhat surprised by that, I've always assumed it would.


It seems everyone assumed it.

> 
> Its sad that mlock() doesn't take a flags argument, so I'd rather
> introduce a new madvise() flag for -rt, something like MADV_UNMOVABLE
> (or whatever) which will basically copy the pages to an un-movable page
> block and really pin the things.


1) We don't have space of vm_flags in 32bit machine and Konstantin
   have sorted out but not sure it's merged. Anyway, Okay. It couldn't be a problem.

2) It needs application's fix and as Mel said, we might get new bug reports about latency.
   Doesn't it break current mlock semantic? - " no page fault can be caused by function calls"
   Otherwise, we should fix man page like your saying -   "no major page fault can be caused by function calls"

 

> That way mlock() can stay what the spec says it is and guarantee
> residency.

> 

> 
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-- 
Kind regards,
Minchan Kim

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