[RFC,0/5] mm/autonuma: replace savedwrite infrastructure

Message

David Hildenbrand Sept. 26, 2022, 3:26 p.m. UTC

As discussed in my talk at LPC, we can reuse the same mechanism for
deciding whether to map a pte writable when upgrading permissions via
mprotect() -- e.g., PROT_READ -> PROT_READ|PROT_WRITE -- to replace the
savedwrite infrastructure used for NUMA hinting faults (e.g., PROT_NONE
-> PROT_READ|PROT_WRITE). Instead of maintaining previous write permissions
for a pte/pmd, we re-determine if the pte/pmd can be writable.

The big benefit is that we have a common logic for deciding whether we can
map a pte/pmd writable on protection changes.

For private mappings, there should be no difference -- from
what I understand, that is what autonuma benchmarks care about.

I ran autonumabench on a system with 2 NUMA nodes, 96 GiB each via:
	perf stat --null --repeat 10

The numa1 benchmark is quite noisy in my environment. I suspect that there
is no actual change in performance, even though the numbers indicate that
this series might improve performance slightly.

numa1:
	mm-stable:   156.75 +- 11.67 seconds time elapsed  ( +-  7.44% )
	mm-stable++: 147.50 +- 9.35 seconds time elapsed  ( +-  6.34% )

numa2:
	mm-stable:   15.9834 +- 0.0589 seconds time elapsed  ( +-  0.37% )
	mm-stable++: 16.1467 +- 0.0946 seconds time elapsed  ( +-  0.59% )

It is worth noting that for shared writable mappings that require
writenotify, we will only avoid write faults if the pte/pmd is dirty
(inherited from the older mprotect logic). If we ever care about optimizing
that further, we'd need a different mechanism to identify whether the FS
still needs to get notified on the next write access. In any case, such an
optimiztion will then not be autonuma-specific, but mprotect() permission
upgrades would similarly benefit from it.

Cc: Linus Torvalds <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Dave Chinner <[e-mail]>
Cc: Nadav Amit <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: Nicholas Piggin <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Anshuman Khandual <[e-mail]>

David Hildenbrand (4):
  mm/mprotect: minor can_change_pte_writable() cleanups
  mm/huge_memory: try avoiding write faults when changing PMD protection
  mm/autonuma: use can_change_(pte|pmd)_writable() to replace savedwrite
  mm: remove unused savedwrite infrastructure

Nadav Amit (1):
  mm/mprotect: allow clean exclusive anon pages to be writable

 arch/powerpc/include/asm/book3s/64/pgtable.h | 80 +-------------------
 arch/powerpc/kvm/book3s_hv_rm_mmu.c          |  2 +-
 include/linux/mm.h                           |  2 +
 include/linux/pgtable.h                      | 24 ------
 mm/debug_vm_pgtable.c                        | 32 --------
 mm/huge_memory.c                             | 66 ++++++++++++----
 mm/ksm.c                                     |  9 +--
 mm/memory.c                                  | 19 ++++-
 mm/mprotect.c                                | 23 +++---
 9 files changed, 93 insertions(+), 164 deletions(-)