On 04/23/2015 02:08 PM, Christian Borntraeger wrote:
Am 23.04.2015 um 14:01 schrieb Alexander Graf:
Am 23.04.2015 um 13:43 schrieb Christian Borntraeger <borntrae...@de.ibm.com>:
Am 23.04.2015 um 13:37 schrieb Alexander Graf:
Am 23.04.2015 um 13:08 schrieb Christian Borntraeger <borntrae...@de.ibm.com>:
From: Martin Schwidefsky <schwidef...@de.ibm.com>
Replacing a 2K page table with a 4K page table while a VMA is active
for the affected memory region is fundamentally broken. Rip out the
page table reallocation code and replace it with a simple system
control 'vm.allocate_pgste'. If the system control is set the page
tables for all processes are allocated as full 4K pages, even for
processes that do not need it.
Signed-off-by: Martin Schwidefsky <schwidef...@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntrae...@de.ibm.com>
Couldn't you make this a hidden kconfig option that gets automatically selected
when kvm is enabled? Or is there a non-kvm case that needs it too?
For things like RHEV the default could certainly be "enabled", but for normal
distros like SLES/RHEL, the idea was to NOT enable that by default, as the
non-KVM
case is more common and might suffer from the additional memory consumption of
the page tables. (big databases come to mind)
We could think about having rpms like kvm to provide a sysctl file that sets it
if we
want to minimize the impact. Other ideas?
Oh, I'm sorry, I misread the ifdef. I don't think it makes sense to have a
config option for the default value then, just rely only on sysctl.conf for
changed defaults.
As far as mechanisms to change it go, every distribution has their own ways of dealing
with this. RH has a "profile" thing, we don't really have anything central, but
individual sysctl.d files for example that a kvm package could provide.
Either way, the default choosing shouldn't happen in .config ;).
So you vote for getting rid of the Kconfig?
Also, please add some helpful error message in qemu to guide users to the
sysctl.
Yes, we will provide a qemu patch (cc stable) after this hits the kernel.
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.
This approach was actually Martins first fix. The problem is that the decision
takes place on execve,
but we need an answer at fork time. So we always started with 4k page tables
and freed the 2nd halv on
execve. Now this did not work for processes that only fork (without execve).
In fact, could we maybe still limit the scope to non-global? A personality may
work as well. Or ulimit?
I think we will go for now with the sysctl and see if we can come up with some
automatic way as additional
patch later on.
Sounds perfectly reasonable to me. You can for example also just set the
sysctl bit in libvirtd :).
Alex
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