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Subject[PATCH RFC 0/5] mm/autonuma: replace savedwrite infrastructure
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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 <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>

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(-)

--
2.37.3

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