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

[Michal Hocko]

On Mon 09-12-19 14:24:22, Logan Gunthorpe wrote:
> 
> 
> 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@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.
> > 
> > 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.

Which looks like something to fix independently.

> 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.

OK, fair enough. Then I would suggest going with arch_add_memory_prot
then unless there is a wider disagreement witht that.
-- 
Michal Hocko
SUSE Labs