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

On 10/22/2014 01:09 PM, Dominik Dingel 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)

... and zero the mm_use_skey flag correctly, too. :)

> v1 -> v2:
> - Following Dave and Paolo suggestion removing the vma flag

Thanks, the patches look good. I expect that they will either go in
through the s390 tree, or come in via Christian.

If the latter, Martin, please reply with your Acked-by.


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