Re: [PATCH][RFC] add MAP_UNLOCKED mmap flag

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

2009/10/6 Gleb Natapov <gleb@redhat.com>:
> On Tue, Oct 06, 2009 at 07:27:56PM +0900, KOSAKI Motohiro wrote:
>> > On Tue, Oct 06, 2009 at 07:11:06PM +0900, KOSAKI Motohiro wrote:
>> > > Hi
>> > >
>> > > > If application does mlockall(MCL_FUTURE) it is no longer possible to
>> > > > mmap file bigger than main memory or allocate big area of anonymous
>> > > > memory. Sometimes it is desirable to lock everything related to program
>> > > > execution into memory, but still be able to mmap big file or allocate
>> > > > huge amount of memory and allow OS to swap them on demand. MAP_UNLOCKED
>> > > > allows to do that.
>> > > >
>> > > > Signed-off-by: Gleb Natapov <gleb@redhat.com>
>> > >
>> > > Why don't you use explicit munlock()?
>> > Because mmap will fail before I'll have a chance to run munlock on it.
>> > Actually when I run my process inside memory limited container host dies
>> > (I suppose trashing, but haven't checked).
>> >
>> > > Plus, Can you please elabrate which workload nedd this feature?
>> > >
>> > I wanted to run kvm with qemu process locked in memory, but guest memory
>> > unlocked. And guest memory is bigger then host memory in the case I am
>> > testing. I found out that it is impossible currently.
>>
>> 1. process creation (qemu)
>> 2. load all library
> Can't control this if program has plugging. Not qemu case
> though.
>
>> 3. mlockall(MCL_CURRENT)
>> 4. load guest OS
> And what about all other allocations qemu does during its life time? Not
> all of them will be small enough to be from brk area.
>
>> is impossible? why?
>>
> Because what you are proposing is not the same as mlockall(MCL_CURRENT|MCL_FUTURE);
>
> You essentially say that MCL_FUTURE is not needed.

No, I only think your case doesn't fit MC_FUTURE.
I haven't find any real benefit in this patch.

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