Re: [PATCH v3 0/4] mm: new function to forbid zeropage mappings for a process

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

On Wed, 22 Oct 2014 13:09:26 +0200
Dominik Dingel <dingel@linux.vnet.ibm.com> wrote:

> s390 has the special notion of storage keys which are some sort of page flags
> associated with physical pages and live outside of direct addressable memory.
> These storage keys can be queried and changed with a special set of instructions.
> The mentioned instructions behave quite nicely under virtualization, if there is: 
> - an invalid pte, then the instructions will work on memory in the host page table
> - a valid pte, then the instructions will work with the real storage key
> 
> Thanks to Martin with his software reference and dirty bit tracking,
> the kernel does not issue any storage key instructions as now a 
> software based approach will be taken, on the other hand distributions 
> in the wild are currently using them.
> 
> However, for virtualized guests we still have a problem with guest pages 
> mapped to zero pages and the kernel same page merging.  
> With each one multiple guest pages will point to the same physical page
> and share the same storage key.
> 
> Let's fix this by introducing a new function which s390 will define to
> forbid new zero page mappings.  If the guest issues a storage key related 
> instruction we flag the mm_struct, drop existing zero page mappings
> and unmerge the guest memory.
> 
> v2 -> v3:
>  - Clearing up patch description Patch 3/4
>  - removing unnecessary flag in mmu_context (Paolo)
> 
> v1 -> v2: 
>  - Following Dave and Paolo suggestion removing the vma flag
> 
> Dominik Dingel (4):
>   s390/mm: recfactor global pgste updates
>   mm: introduce mm_forbids_zeropage function
>   s390/mm: prevent and break zero page mappings in case of storage keys
>   s390/mm: disable KSM for storage key enabled pages
> 
>  arch/s390/include/asm/pgalloc.h |   2 -
>  arch/s390/include/asm/pgtable.h |   8 +-
>  arch/s390/kvm/kvm-s390.c        |   2 +-
>  arch/s390/kvm/priv.c            |  17 ++--
>  arch/s390/mm/pgtable.c          | 180 ++++++++++++++++++----------------------
>  include/linux/mm.h              |   4 +
>  mm/huge_memory.c                |   2 +-
>  mm/memory.c                     |   2 +-
>  8 files changed, 106 insertions(+), 111 deletions(-)
 
Patches look good to me and as nobody seems to disagree with the proposed
solution I will add the code to the features branch of the s390 tree.

-- 
blue skies,
   Martin.

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

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