Re: [PATCH v3 6/7] mm/memory_hotplug: Add pgprot_t to mhp_params

[Dan Williams]

On Mon, Mar 2, 2020 at 10:55 AM Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:
>
>
>
> On 2020-02-29 3:44 p.m., Dan Williams wrote:
> > On Fri, Feb 21, 2020 at 10:25 AM Logan Gunthorpe <logang@xxxxxxxxxxxx> 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, provide a way to specify the pgprot value explicitly to
> >> arch_add_memory().
> >>
> >> Of the arches that support MEMORY_HOTPLUG: x86_64, and arm64 need 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 ia64, s390 and
> >> sh, reject anything but PAGE_KERNEL settings -- this should be fine,
> >> for now, seeing these architectures don't support ZONE_DEVICE.
> >>
> >> A check in __add_pages() is also added to ensure the pgprot parameter was
> >> set for all arches.
> >>
> >> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> >> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> >> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> >> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> >> ---
> >>  arch/arm64/mm/mmu.c            | 3 ++-
> >>  arch/ia64/mm/init.c            | 3 +++
> >>  arch/powerpc/mm/mem.c          | 3 ++-
> >>  arch/s390/mm/init.c            | 3 +++
> >>  arch/sh/mm/init.c              | 3 +++
> >>  arch/x86/mm/init_32.c          | 5 +++++
> >>  arch/x86/mm/init_64.c          | 2 +-
> >>  include/linux/memory_hotplug.h | 2 ++
> >>  mm/memory_hotplug.c            | 5 ++++-
> >>  mm/memremap.c                  | 6 +++---
> >>  10 files changed, 28 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> >> index ee37bca8aba8..ea3fa844a8a2 100644
> >> --- a/arch/arm64/mm/mmu.c
> >> +++ b/arch/arm64/mm/mmu.c
> >> @@ -1058,7 +1058,8 @@ int arch_add_memory(int nid, u64 start, u64 size,
> >>                 flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> >>
> >>         __create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
> >> -                            size, PAGE_KERNEL, __pgd_pgtable_alloc, flags);
> >> +                            size, params->pgprot, __pgd_pgtable_alloc,
> >> +                            flags);
> >>
> >>         memblock_clear_nomap(start, size);
> >>
> >> diff --git a/arch/ia64/mm/init.c b/arch/ia64/mm/init.c
> >> index 97bbc23ea1e3..d637b4ea3147 100644
> >> --- a/arch/ia64/mm/init.c
> >> +++ b/arch/ia64/mm/init.c
> >> @@ -676,6 +676,9 @@ int arch_add_memory(int nid, u64 start, u64 size,
> >>         unsigned long nr_pages = size >> PAGE_SHIFT;
> >>         int ret;
> >>
> >> +       if (WARN_ON_ONCE(params->pgprot.pgprot != PAGE_KERNEL.pgprot))
> >> +               return -EINVAL;
> >> +
> >>         ret = __add_pages(nid, start_pfn, nr_pages, params);
> >>         if (ret)
> >>                 printk("%s: Problem encountered in __add_pages() as ret=%d\n",
> >> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> >> index 19b1da5d7eca..832412bc7fad 100644
> >> --- a/arch/powerpc/mm/mem.c
> >> +++ b/arch/powerpc/mm/mem.c
> >> @@ -138,7 +138,8 @@ int __ref arch_add_memory(int nid, u64 start, u64 size,
> >>         resize_hpt_for_hotplug(memblock_phys_mem_size());
> >>
> >>         start = (unsigned long)__va(start);
> >> -       rc = create_section_mapping(start, start + size, nid, PAGE_KERNEL);
> >> +       rc = create_section_mapping(start, start + size, nid,
> >> +                                   params->pgprot);
> >>         if (rc) {
> >>                 pr_warn("Unable to create mapping for hot added memory 0x%llx..0x%llx: %d\n",
> >>                         start, start + size, rc);
> >> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
> >> index e9e4a7abd0cc..87b2d024e75a 100644
> >> --- a/arch/s390/mm/init.c
> >> +++ b/arch/s390/mm/init.c
> >> @@ -277,6 +277,9 @@ int arch_add_memory(int nid, u64 start, u64 size,
> >>         if (WARN_ON_ONCE(params->altmap))
> >>                 return -EINVAL;
> >>
> >> +       if (WARN_ON_ONCE(params->pgprot.pgprot != PAGE_KERNEL.pgprot))
> >> +               return -EINVAL;
> >> +
> >>         rc = vmem_add_mapping(start, size);
> >>         if (rc)
> >>                 return rc;
> >> diff --git a/arch/sh/mm/init.c b/arch/sh/mm/init.c
> >> index e5114c053364..b9de2d4fa57e 100644
> >> --- a/arch/sh/mm/init.c
> >> +++ b/arch/sh/mm/init.c
> >> @@ -412,6 +412,9 @@ int arch_add_memory(int nid, u64 start, u64 size,
> >>         unsigned long nr_pages = size >> PAGE_SHIFT;
> >>         int ret;
> >>
> >> +       if (WARN_ON_ONCE(params->pgprot.pgprot != PAGE_KERNEL.pgprot)
> >> +               return -EINVAL;
> >> +
> >>         /* We only have ZONE_NORMAL, so this is easy.. */
> >>         ret = __add_pages(nid, start_pfn, nr_pages, params);
> >>         if (unlikely(ret))
> >> diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
> >> index e25a4218e6ff..96d8e4fb1cc8 100644
> >> --- a/arch/x86/mm/init_32.c
> >> +++ b/arch/x86/mm/init_32.c
> >> @@ -858,6 +858,11 @@ int arch_add_memory(int nid, u64 start, u64 size,
> >>  {
> >>         unsigned long start_pfn = start >> PAGE_SHIFT;
> >>         unsigned long nr_pages = size >> PAGE_SHIFT;
> >> +       int ret;
> >> +
> >> +       ret = _set_memory_prot(start, nr_pages, params->pgprot);
> >
> > Perhaps a comment since it's not immediately obvious where the
> > PAGE_KERNEL prot was established, and perhaps add a conditional to
> > skip this call in the param->pgprot == PAGE_KERNEL case?
>
> Yes I can add the skip in the PAGE_KERNEL case. Though I'm not sure what
> you are asking for with regards to the comment. Just that pgprot is set
> by the caller usually to PAGE_KERNEL?

No, I'm reacting to this comment in the changelog "For x86_32, set the
page tables explicitly using _set_memory_prot() (seeing they are
already mapped)". You've done some investigation that
x86_32::arch_add_memory() expects the page tables to be already
established. I think that's worth capturing inline in the code for
other people doing cross-arch arch_add_memory() changes.