Re: [PATCH] KVM: s390: remove delayed reallocation of page tables for KVM

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Mon, 27 Apr 2015 15:48:42 +0200
Alexander Graf <agraf@suse.de> wrote:

> On 04/23/2015 02:13 PM, Martin Schwidefsky wrote:
> > On Thu, 23 Apr 2015 14:01:23 +0200
> > Alexander Graf <agraf@suse.de> wrote:
> >
> >> As far as alternative approaches go, I don't have a great idea otoh.
> >> We could have an elf flag indicating that this process needs 4k page
> >> tables to limit the impact to a single process. In fact, could we
> >> maybe still limit the scope to non-global? A personality may work
> >> as well. Or ulimit?
> > I tried the ELF flag approach, does not work. The trouble is that
> > allocate_mm() has to create the page tables with 4K tables if you
> > want to change the page table layout later on. We have learned the
> > hard way that the direction 2K to 4K does not work due to races
> > in the mm.
> >
> > Now there are two major cases: 1) fork + execve and 2) fork only.
> > The ELF flag can be used to reduce from 4K to 2K for 1) but not 2).
> > 2) is required for apps that use lots of forking, e.g. database or
> > web servers. Same goes for the approach with a personality flag or
> > ulimit.
> >
> > We would have to distinguish the two cases for allocate_mm(),
> > if the new mm is allocated for a fork the current mm decides
> > 2K vs. 4K. If the new mm is allocated by binfmt_elf, then start
> > with 4K and do the downgrade after the ELF flag has been evaluated.
> 
> Well, you could also make it a personality flag for example, no? Then 
> every new process below a certain one always gets 4k page tables until 
> they drop the personality, at which point each child would only get 2k 
> page tables again.
> 
> I'm mostly concerned that people will end up mixing VMs and other 
> workloads on the same LPAR, so I don't think there's a one-shoe-fits-all 
> solution.

If I add an argument to mm_init() to indicate if this context
is for fork() or execve() then the ELF header flag approach works.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.