[PATCH v4 0/7] Allow setting caching mode in arch_add_memory() for P2PDMA

[Logan Gunthorpe]

Hi,

This is v4 of the patchset which cleans up a number of minor issues
from the feedback of v3 and rebases onto v5.6-rc4. Additional feedback
is welcome.

Also worth noting, is that the kernel test robot reports[1] that Patch 3
in this series improves will-it-scale.per_process_ops by 36%. Though,
for the life of me, I can't understand why that would be. But it's
reported the same thing twice now for different versions of the series.

Thanks,

Logan

[1] https://lists.01.org/hyperkitty/list/lkp@lists.01.org/thread/5APDKNBEJGVJTJRTI2IIA3P4OC2OEYPS/

--

Changes in v4:
 * Rebased onto v5.6-rc4
 * Collected tags form David, Dan and Michal
 * Minor changes to the new _set_memory_prot() function and added some
   comments as requested by Dan.
 * Changed the default caching type for P2PDMA memory to UC instead of
   WC per Jason's concerns that WC might be more generally unsafe.

Changes in v3:
 * Rebased onto v5.6-rc2
 * Rename mhp_modifiers to mhp_params per David with an updated kernel
   doc per Dan
 * Drop support for s390 per David seeing it does not support
   ZONE_DEVICE yet and there was a potential problem with huge pages.
 * Added WARN_ON_ONCE in cases where arches recieve non PAGE_KERNEL
   parameters
 * Collected David and Micheal's Reviewed-By and Acked-by Tags

Changes in v2:
 * Rebased onto v5.5-rc5
 * Renamed mhp_restrictions to mhp_modifiers and added the pgprot field
   to that structure instead of using an argument for
   arch_add_memory().
 * Add patch to drop the unused flags field in mhp_restrictions

A git branch is available here:

https://github.com/sbates130272/linux-p2pmem remap_pages_cache_v4

--

Currently, the page tables created using memremap_pages() are always
created with the PAGE_KERNEL cacheing mode. However, the P2PDMA code
is creating pages for PCI BAR memory which should never be accessed
through the cache and instead use either WC or UC. This still works in
most cases, on x86, because the MTRR registers typically override the
caching settings in the page tables for all of the IO memory to be
UC-. However, this tends not to work so well on other arches or
some rare x86 machines that have firmware which does not setup the
MTRR registers in this way.

Instead of this, this series proposes a change to arch_add_memory()
to take the pgprot required by the mapping which allows us to
explicitly set pagetable entries for P2PDMA memory to UC.

This changes is pretty routine for most of the arches: x86_64, arm64
and powerpc simply need to thread the pgprot through to where the page
tables are setup. x86_32 unfortunately sets up the page tables at boot so
must use _set_memory_prot() to change their caching mode. ia64, s390 and sh
don't appear to have an easy way to change the page tables so, for now
at least, we just return -EINVAL on such mappings and thus they will
not support P2PDMA memory until the work for this is done. This should
be fine as they don't yet support ZONE_DEVICE.

--

Logan Gunthorpe (7):
  mm/memory_hotplug: Drop the flags field from struct mhp_restrictions
  mm/memory_hotplug: Rename mhp_restrictions to mhp_params
  x86/mm: Thread pgprot_t through init_memory_mapping()
  x86/mm: Introduce __set_memory_prot()
  powerpc/mm: Thread pgprot_t through create_section_mapping()
  mm/memory_hotplug: Add pgprot_t to mhp_params
  mm/memremap: Set caching mode for PCI P2PDMA memory to WC

 arch/arm64/mm/mmu.c                        |  7 ++--
 arch/ia64/mm/init.c                        |  7 ++--
 arch/powerpc/include/asm/book3s/64/hash.h  |  3 +-
 arch/powerpc/include/asm/book3s/64/radix.h |  3 +-
 arch/powerpc/include/asm/sparsemem.h       |  3 +-
 arch/powerpc/mm/book3s64/hash_utils.c      |  5 +--
 arch/powerpc/mm/book3s64/pgtable.c         |  7 ++--
 arch/powerpc/mm/book3s64/radix_pgtable.c   | 18 ++++++----
 arch/powerpc/mm/mem.c                      | 10 +++---
 arch/s390/mm/init.c                        |  9 +++--
 arch/sh/mm/init.c                          |  7 ++--
 arch/x86/include/asm/page_types.h          |  3 --
 arch/x86/include/asm/pgtable.h             |  3 ++
 arch/x86/include/asm/set_memory.h          |  1 +
 arch/x86/kernel/amd_gart_64.c              |  3 +-
 arch/x86/mm/init.c                         |  9 ++---
 arch/x86/mm/init_32.c                      | 19 ++++++++--
 arch/x86/mm/init_64.c                      | 40 ++++++++++++----------
 arch/x86/mm/mm_internal.h                  |  3 +-
 arch/x86/mm/pat/set_memory.c               | 13 +++++++
 arch/x86/platform/uv/bios_uv.c             |  3 +-
 include/linux/memory_hotplug.h             | 21 ++++++------
 mm/memory_hotplug.c                        | 11 +++---
 mm/memremap.c                              | 17 +++++----
 24 files changed, 144 insertions(+), 81 deletions(-)


base-commit: 98d54f81e36ba3bf92172791eba5ca5bd813989b
--
2.20.1