Re: [PATCH v6] add MAP_UNLOCKED mmap flag

From: Gleb Natapov
Date: Tue Jan 19 2010 - 03:27:44 EST

On Tue, Jan 19, 2010 at 10:07:07AM +0200, Pekka Enberg wrote:
> Hi Gleb,
> On Tue, Jan 19, 2010 at 9:52 AM, Gleb Natapov <gleb@xxxxxxxxxx> wrote:
> >> It would be probably useful if you could point us to the application
> >> source code that actually wants this feature.
> >>
> > This is two line patch to qemu that calls mlockall(MCL_CURRENT|MCL_FUTURE)
> > at the beginning of the main() and changes guest memory allocation to
> > use MAP_UNLOCKED flag. All alternative solutions in this thread suggest
> > that I should rewrite qemu + all library it uses. You see why I can't
> > take them seriously?
> Well, that's not going to be portable, is it, so the application
KVM is not portable ;) and that is what my main interest is.

> design would still be broken, no? Did you try using (or extending)
> posix_madvise(MADV_DONTNEED) for the guest address space? It seems to
After mlockall() I can't even allocate guest address space. Or do you mean
instead of mlockall()? Then how MADV_DONTNEED will help? It just drops
page table for the address range (which is not what I need) and does not
have any long time effect.

> me that you're trying to use a big hammer (mlock) when a polite hint
> for the VM would probably be sufficient for it do its job.
I what to tell to VM "swap this, don't swap that" and as far as I see
there is no other way to do it currently.

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