Merge the lockless page table walk rework into next

This merges the lockless page table walk rework series from Aneesh.
Because it touches powerpc KVM code we are sharing it with the kvm-ppc
tree in our topic/ppc-kvm branch.

This is the cover letter from Aneesh:

Avoid IPI while updating page table entries.

Problem Summary:
Slow termination of KVM guest with large guest RAM config due to a
large number of IPIs that were caused by clearing level 1 PTE
entries (THP) entries. This is shown in the stack trace below.

- qemu-system-ppc  [kernel.vmlinux]            [k] smp_call_function_many
   - smp_call_function_many
      - 36.09% smp_call_function_many
           serialize_against_pte_lookup
           radix__pmdp_huge_get_and_clear
           zap_huge_pmd
           unmap_page_range
           unmap_vmas
           unmap_region
           __do_munmap
           __vm_munmap
           sys_munmap
          system_call
           __munmap
           qemu_ram_munmap
           qemu_anon_ram_free
           reclaim_ramblock
           call_rcu_thread
           qemu_thread_start
           start_thread
           __clone

Why we need to do IPI when clearing PMD entries:
This was added as part of commit: 13bd817bb884 ("powerpc/thp: Serialize pmd clear against a linux page table walk")

serialize_against_pte_lookup makes sure that all parallel lockless
page table walk completes before we convert a PMD pte entry to regular
pmd entry. We end up doing that conversion in the below scenarios

1) __split_huge_zero_page_pmd
2) do_huge_pmd_wp_page_fallback
3) MADV_DONTNEED running parallel to page faults.

local_irq_disable and lockless page table walk:

The lockless page table walk work with the assumption that we can
dereference the page table contents without holding a lock. For this
to work, we need to make sure we read the page table contents
atomically and page table pages are not going to be freed/released
while we are walking the table pages. We can achieve by using a rcu
based freeing for page table pages or if the architecture implements
broadcast tlbie, we can block the IPI as we walk the page table pages.

To support both the above framework, lockless page table walk is done
with irq disabled instead of rcu_read_lock()

We do have two interface for lockless page table walk, gup fast and
__find_linux_pte. This patch series makes __find_linux_pte table walk
safe against the conversion of PMD PTE to regular PMD.

gup fast:

gup fast is already safe against THP split because kernel now
differentiate between a pmd split and a compound page split. gup fast
can run parallel to a pmd split and we prevent a parallel gup fast to
a hugepage split, by freezing the page refcount and failing the
speculative page ref increment.

Similar to how gup is safe against parallel pmd split, this patch
series updates the __find_linux_pte callers to be safe against a
parallel pmd split. We do that by enforcing the following rules.

1) Don't reload the pte value, because that can be updated in
   parallel.
2) Code should be able to work with a stale PTE value and not the
   recent one. ie, the pte value that we are looking at may not be the
   latest value in the page table.
3) Before looking at pte value check for _PAGE_PTE bit. We now do this
as part of pte_present() check.

Performance:

This speeds up Qemu guest RAM del/unplug time as below
128 core, 496GB guest:

Without patch:
  munmap start: timer = 13162 ms, PID=7684
  munmap finish: timer = 95312 ms, PID=7684 - delta = 82150 ms

With patch (upto removing IPI)
  munmap start: timer = 196449 ms, PID=6681
  munmap finish: timer = 196488 ms, PID=6681 - delta = 39ms

With patch (with adding the tlb invalidate in pmdp_huge_get_and_clear_full)
  munmap start: timer = 196345 ms, PID=6879
  munmap finish: timer = 196714 ms, PID=6879 - delta = 369ms

Link: https://lore.kernel.org/r/20200505071729.54912-1-aneesh.kumar@linux.ibm.com

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