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

[Logan Gunthorpe]

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.

Thanks,

Logan