Re: [PATCH 5/6] mm, memory_hotplug: Provide argument for the pgprot_t in arch_add_memory()

[Dan Williams]

On Mon, Dec 9, 2019 at 12:24 PM Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:
>
>
>
> On 2019-12-09 12:23 p.m., David Hildenbrand wrote:
> > On 09.12.19 20:13, Logan Gunthorpe wrote:
> >> devm_memremap_pages() is currently used by the PCI P2PDMA code to create
> >> struct page mappings for IO memory. At present, these mappings are created
> >> with PAGE_KERNEL which implies setting the PAT bits to be WB. However, on
> >> x86, an mtrr register will typically override this and force the cache
> >> type to be UC-. In the case firmware doesn't set this register it is
> >> effectively WB and will typically result in a machine check exception
> >> when it's accessed.
> >>
> >> Other arches are not currently likely to function correctly seeing they
> >> don't have any MTRR registers to fall back on.
> >>
> >> To solve this, add an argument to arch_add_memory() to explicitly
> >> set the pgprot value to a specific value.
> >>
> >> Of the arches that support MEMORY_HOTPLUG: x86_64, s390 and arm64 is a
> >> simple change to pass the pgprot_t down to their respective functions
> >> which set up the page tables. For x86_32, set the page tables explicitly
> >> using _set_memory_prot() (seeing they are already mapped). For sh, reject
> >> anything but PAGE_KERNEL settings -- this should be fine, for now, seeing
> >> sh doesn't support ZONE_DEVICE anyway.
> >>
> >> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> >> Cc: David Hildenbrand <david@xxxxxxxxxx>
> >> Cc: Michal Hocko <mhocko@xxxxxxxx>
> >> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> >> ---
> >>  arch/arm64/mm/mmu.c            | 4 ++--
> >>  arch/ia64/mm/init.c            | 5 ++++-
> >>  arch/powerpc/mm/mem.c          | 4 ++--
> >>  arch/s390/mm/init.c            | 4 ++--
> >>  arch/sh/mm/init.c              | 5 ++++-
> >>  arch/x86/mm/init_32.c          | 7 ++++++-
> >>  arch/x86/mm/init_64.c          | 4 ++--
> >>  include/linux/memory_hotplug.h | 2 +-
> >>  mm/memory_hotplug.c            | 2 +-
> >>  mm/memremap.c                  | 2 +-
> >>  10 files changed, 25 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> >> index 60c929f3683b..48b65272df15 100644
> >> --- a/arch/arm64/mm/mmu.c
> >> +++ b/arch/arm64/mm/mmu.c
> >> @@ -1050,7 +1050,7 @@ int p4d_free_pud_page(p4d_t *p4d, unsigned long addr)
> >>  }
> >>
> >>  #ifdef CONFIG_MEMORY_HOTPLUG
> >> -int arch_add_memory(int nid, u64 start, u64 size,
> >> +int arch_add_memory(int nid, u64 start, u64 size, pgprot_t prot,
> >>                      struct mhp_restrictions *restrictions)
> >
> > Can we fiddle that into "struct mhp_restrictions" instead?
>
> Yes, if that's what people want, it's pretty trivial to do. I chose not
> to do it that way because it doesn't get passed down to add_pages() and
> it's not really a "restriction". If I don't hear any objections, I will
> do that for v2.

+1 to storing this information alongside the altmap in that structure.
However, I agree struct mhp_restrictions, with the MHP_MEMBLOCK_API
flag now gone, has lost all of its "restrictions". How about dropping
the 'flags' property and renaming the struct to 'struct
mhp_modifiers'?