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

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 23.04.2015 um 13:37 schrieb Alexander Graf:
> 
> 
>> Am 23.04.2015 um 13:08 schrieb Christian Borntraeger <borntraeger@de.ibm.com>:
>>
>> From: Martin Schwidefsky <schwidefsky@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 <schwidefsky@de.ibm.com>
>> Signed-off-by: Christian Borntraeger <borntraeger@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?

Christian