Re: [PATCH V3 0/4] Fix kvm misconceives NVDIMM pages as reserved mmio

[David Hildenbrand <david@redhat.com>]

On 09.08.2018 12:52, Zhang Yi wrote:
> For device specific memory space, when we move these area of pfn to
> memory zone, we will set the page reserved flag at that time, some of
> these reserved for device mmio, and some of these are not, such as
> NVDIMM pmem.
> 
> Now, we map these dev_dax or fs_dax pages to kvm for DIMM/NVDIMM
> backend, since these pages are reserved. the check of
> kvm_is_reserved_pfn() misconceives those pages as MMIO. Therefor, we
> introduce 2 page map types, MEMORY_DEVICE_FS_DAX/MEMORY_DEVICE_DEV_DAX,
> to indentify these pages are from NVDIMM pmem. and let kvm treat these
> as normal pages.
> 
> Without this patch, Many operations will be missed due to this
> mistreatment to pmem pages. For example, a page may not have chance to
> be unpinned for KVM guest(in kvm_release_pfn_clean); not able to be
> marked as dirty/accessed(in kvm_set_pfn_dirty/accessed) etc.
> 

I am right now looking into (and trying to better document) PG_reserved
- and having a hard time :) .

One of the main points about reserved pages is that the struct pages are
not to be touched. See [1] (I know that statement is fairly old, but it
resembles what PG_reserved is actually used for nowadays - with some
exceptions unfortunately.).

Struct pages part of user space tables that are PG_reserved can indicate
(as of now according to my research)
- MMIO pages
- Selected MMAPed pages - e.g. vDSO
- Zero page
- PMEM pages as you correctly state

So I wonder, if it is really the right approach to silently go ahead and
treat reserved pages just like they would not be reserved. Maybe the
right approach would rather be to do something about pmem pages being
reserved. Yes, they are never to be given to the page allocator, but I
wonder if PG_reserved is strictly needed for that.

[1] https://lists.linuxcoding.com/kernel/2005-q3/msg10350.html

> V1:
> https://lkml.org/lkml/2018/7/4/91
> 
> V2:
> https://lkml.org/lkml/2018/7/10/135
> 
> V3:
> [PATCH V3 1/4] Needs Comments.
> [PATCH V3 2/4] Update the description of MEMORY_DEVICE_DEV_DAX: Jan
> [PATCH V3 3/4] Acked-by: Jan in V2
> [PATCH V3 4/4] Needs Comments.
> 
> Zhang Yi (4):
>   kvm: remove redundant reserved page check
>   mm: introduce memory type MEMORY_DEVICE_DEV_DAX
>   mm: add a function to differentiate the pages is from DAX device
>     memory
>   kvm: add a check if pfn is from NVDIMM pmem.
> 
>  drivers/dax/pmem.c       |  1 +
>  include/linux/memremap.h |  8 ++++++++
>  include/linux/mm.h       | 12 ++++++++++++
>  virt/kvm/kvm_main.c      | 16 ++++++++--------
>  4 files changed, 29 insertions(+), 8 deletions(-)
> 


-- 

Thanks,

David / dhildenb