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

From: Martin Schwidefsky
Date: Thu Oct 23 2014 - 06:20:04 EST


On Wed, 22 Oct 2014 13:09:26 +0200
Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx> 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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