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

[Logan Gunthorpe <logang@deltatee.com>]

On 2019-12-09 1:41 p.m., Michal Hocko wrote:
> On Mon 09-12-19 13:24:19, Logan Gunthorpe 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.
> 
> I do agree that restriction is not the best fit. But I consider prot
> argument to complicate the API to all users even though it is not really
> clear whether we are going to have many users really benefiting from it.
> Look at the vmalloc API and try to find how many users of __vmalloc do
> not use PAGE_KERNEL.
> 
> So I can see two options. One of them is to add arch_add_memory_prot
> that would allow to have give and extra prot argument or simply call
> an arch independent API to change the protection after arch_add_memory.
> The later sounds like much less code. The memory shouldn't be in use by
> anybody at that stage yet AFAIU. Maybe there even is an API like that.

Yes, well, we tried something like this by calling set_memory_wc()
inside memremap_pages(); but on large bars (tens of GB) it was too slow
(taking several seconds to complete) and on some hosts actually hit CPU
watchdog errors.

So at the very least we'd have to add some cpu_relax() calls to that
path. And it's also the case that set_memory_wc() is x86 only right now.
So we'd have to create a new general interface to walk and fixup page
tables for all arches.

But, in my opinion, setting up all those page tables twice is too large
of an overhead and it's better to just add them correctly the first
time. The changes I propose to do this aren't really a lot of code and
probably less than creating a new interface for all arches.

Logan