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

Logan Gunthorpe
2019-12-09T20:24:19Z

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.

Thanks,

Logan