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

[David Hildenbrand <david@redhat.com>]

> Am 09.12.2019 um 21:43 schrieb Dan Williams <dan.j.williams@intel.com>:
> 
> On Mon, Dec 9, 2019 at 12:24 PM Logan Gunthorpe <logang@deltatee.com> 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@intel.com>
>>>> Cc: David Hildenbrand <david@redhat.com>
>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
>>>> ---
>>>> 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'?
> 

I‘d prefer that over an arch_add_memory_protected() as suggested by Michal. But if we can change it after adding the memory (as also suggested by Michal), that would also be nice.

Cheers!