[Xen-devel] [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)

[David Hildenbrand <david@xxxxxxxxxx>]

This series is based on [2], which should pop up in linux/next soon:
        https://lkml.org/lkml/2019/10/21/1034

This is the result of a recent discussion with Michal ([1], [2]). Right
now we set all pages PG_reserved when initializing hotplugged memmaps. This
includes ZONE_DEVICE memory. In case of system memory, PG_reserved is
cleared again when onlining the memory, in case of ZONE_DEVICE memory
never. In ancient times, we needed PG_reserved, because there was no way
to tell whether the memmap was already properly initialized. We now have
SECTION_IS_ONLINE for that in the case of !ZONE_DEVICE memory. ZONE_DEVICE
memory is already initialized deferred, and there shouldn't be a visible
change in that regard.

I remember that some time ago, we already talked about stopping to set
ZONE_DEVICE pages PG_reserved on the list, but I never saw any patches.
Also, I forgot who was part of the discussion :)

One of the biggest fear were side effects. I went ahead and audited all
users of PageReserved(). The ones that don't need any care (patches)
can be found below. I will double check and hope I am not missing something
important.

I am probably a little bit too careful (but I don't want to break things).
In most places (besides KVM and vfio that are nuts), the
pfn_to_online_page() check could most probably be avoided by a
is_zone_device_page() check. However, I usually get suspicious when I see
a pfn_valid() check (especially after I learned that people mmap parts of
/dev/mem into user space, including memory without memmaps. Also, people
could memmap offline memory blocks this way :/). As long as this does not
hurt performance, I think we should rather do it the clean way.

I only gave it a quick test with DIMMs on x86-64, but didn't test the
ZONE_DEVICE part at all (any tips for a nice QEMU setup?). Compile-tested
on x86-64 and PPC.

Other users of PageReserved() that should be fine:
- mm/page_owner.c:pagetypeinfo_showmixedcount_print()
  -> Never called for ZONE_DEVICE, (+ pfn_to_online_page(pfn))
- mm/page_owner.c:init_pages_in_zone()
  -> Never called for ZONE_DEVICE (!populated_zone(zone))
- mm/page_ext.c:free_page_ext()
  -> Only a BUG_ON(PageReserved(page)), not relevant
- mm/page_ext.c:has_unmovable_pages()
  -> Not releveant for ZONE_DEVICE
- mm/page_ext.c:pfn_range_valid_contig()
  -> pfn_to_online_page() already guards us
- mm/mempolicy.c:queue_pages_pte_range()
  -> vm_normal_page() checks against pte_devmap()
- mm/memory-failure.c:hwpoison_user_mappings()
  -> Not reached via memory_failure() due to pfn_to_online_page()
  -> Also not reached indirectly via memory_failure_hugetlb()
- mm/hugetlb.c:gather_bootmem_prealloc()
  -> Only a WARN_ON(PageReserved(page)), not relevant
- kernel/power/snapshot.c:saveable_highmem_page()
  -> pfn_to_online_page() already guards us
- kernel/power/snapshot.c:saveable_page()
  -> pfn_to_online_page() already guards us
- fs/proc/task_mmu.c:can_gather_numa_stats()
  -> vm_normal_page() checks against pte_devmap()
- fs/proc/task_mmu.c:can_gather_numa_stats_pmd
  -> vm_normal_page_pmd() checks against pte_devmap()
- fs/proc/page.c:stable_page_flags()
  -> The reserved bit is simply copied, irrelevant
- drivers/firmware/memmap.c:release_firmware_map_entry()
  -> really only a check to detect bootmem. Not relevant for ZONE_DEVICE
- arch/ia64/kernel/mca_drv.c
- arch/mips/mm/init.c
- arch/mips/mm/ioremap.c
- arch/nios2/mm/ioremap.c
- arch/parisc/mm/ioremap.c
- arch/sparc/mm/tlb.c
- arch/xtensa/mm/cache.c
  -> No ZONE_DEVICE support
- arch/powerpc/mm/init_64.c:vmemmap_free()
  -> Special-cases memmap on altmap
  -> Only a check for bootmem
- arch/x86/kernel/alternative.c:__text_poke()
  -> Only a WARN_ON(!PageReserved(pages[0])) to verify it is bootmem
- arch/x86/mm/init_64.c
  -> Only a check for bootmem

[1] https://lkml.org/lkml/2019/10/21/736
[2] https://lkml.org/lkml/2019/10/21/1034

Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: kvm-ppc@xxxxxxxxxxxxxxx
Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
Cc: kvm@xxxxxxxxxxxxxxx
Cc: linux-hyperv@xxxxxxxxxxxxxxx
Cc: devel@xxxxxxxxxxxxxxxxxxxx
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: x86@xxxxxxxxxx
Cc: Alexander Duyck <alexander.duyck@xxxxxxxxx>

David Hildenbrand (12):
  mm/memory_hotplug: Don't allow to online/offline memory blocks with
    holes
  mm/usercopy.c: Prepare check_page_span() for PG_reserved changes
  KVM: x86/mmu: Prepare kvm_is_mmio_pfn() for PG_reserved changes
  KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes
  vfio/type1: Prepare is_invalid_reserved_pfn() for PG_reserved changes
  staging/gasket: Prepare gasket_release_page() for PG_reserved changes
  staging: kpc2000: Prepare transfer_complete_cb() for PG_reserved
    changes
  powerpc/book3s: Prepare kvmppc_book3s_instantiate_page() for
    PG_reserved changes
  powerpc/64s: Prepare hash_page_do_lazy_icache() for PG_reserved
    changes
  powerpc/mm: Prepare maybe_pte_to_page() for PG_reserved changes
  x86/mm: Prepare __ioremap_check_ram() for PG_reserved changes
  mm/memory_hotplug: Don't mark pages PG_reserved when initializing the
    memmap

 arch/powerpc/kvm/book3s_64_mmu_radix.c     | 14 ++++---
 arch/powerpc/mm/book3s64/hash_utils.c      | 10 +++--
 arch/powerpc/mm/pgtable.c                  | 10 +++--
 arch/x86/kvm/mmu.c                         | 30 +++++++++------
 arch/x86/mm/ioremap.c                      | 13 +++++--
 drivers/hv/hv_balloon.c                    |  6 +++
 drivers/staging/gasket/gasket_page_table.c |  2 +-
 drivers/staging/kpc2000/kpc_dma/fileops.c  |  3 +-
 drivers/vfio/vfio_iommu_type1.c            | 10 ++++-
 drivers/xen/balloon.c                      |  7 ++++
 include/linux/page-flags.h                 |  8 +---
 mm/memory_hotplug.c                        | 43 ++++++++++++++++------
 mm/page_alloc.c                            | 11 ------
 mm/usercopy.c                              |  5 ++-
 virt/kvm/kvm_main.c                        | 10 ++++-
 15 files changed, 115 insertions(+), 67 deletions(-)

-- 
2.21.0


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