linux-kernel - [PATCH RFC 0/5] mm/autonuma: replace savedwrite infrastructure

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Date:   Mon, 26 Sep 2022 17:26:13 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        David Hildenbrand <david@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Dave Chinner <david@...morbit.com>,
        Nadav Amit <namit@...are.com>, Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: [PATCH RFC 0/5] mm/autonuma: replace savedwrite infrastructure

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@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Mel Gorman <mgorman@...hsingularity.net>
Cc: Dave Chinner <david@...morbit.com>
Cc: Nadav Amit <namit@...are.com>
Cc: Peter Xu <peterx@...hat.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: Mike Rapoport <rppt@...nel.org>
Cc: Anshuman Khandual <anshuman.khandual@....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