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

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

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.

-- 
blue skies,
   Martin.

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