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Message-ID: <20241206225204.4008261-1-surenb@google.com>
Date: Fri, 6 Dec 2024 14:51:57 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: akpm@...ux-foundation.org
Cc: willy@...radead.org, liam.howlett@...cle.com, lorenzo.stoakes@...cle.com,
mhocko@...e.com, vbabka@...e.cz, hannes@...xchg.org, mjguzik@...il.com,
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souravpanda@...gle.com, pasha.tatashin@...een.com, corbet@....net,
linux-doc@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
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Subject: [PATCH v5 0/6] move per-vma lock into vm_area_struct
Back when per-vma locks were introduces, vm_lock was moved out of
vm_area_struct in [1] because of the performance regression caused by
false cacheline sharing. Recent investigation [2] revealed that the
regressions is limited to a rather old Broadwell microarchitecture and
even there it can be mitigated by disabling adjacent cacheline
prefetching, see [3].
Splitting single logical structure into multiple ones leads to more
complicated management, extra pointer dereferences and overall less
maintainable code. When that split-away part is a lock, it complicates
things even further. With no performance benefits, there are no reasons
for this split. Merging the vm_lock back into vm_area_struct also allows
vm_area_struct to use SLAB_TYPESAFE_BY_RCU later in this patchset.
This patchset:
1. moves vm_lock back into vm_area_struct, aligning it at the cacheline
boundary and changing the cache to be cacheline-aligned to minimize
cacheline sharing;
2. changes vm_area_struct initialization to mark new vma as detached until
it is inserted into vma tree;
3. changes vm_area_struct cache to SLAB_TYPESAFE_BY_RCU to allow for their
reuse and to minimize call_rcu() calls. To avoid bloating vm_area_struct,
we introduce unioned freeptr_t field and allow freeptr_offset to be used
with ctor in a separate patch.
Pagefault microbenchmarks do not show noticeable performance change.
Changes since v4 [4]
- Added SOBs, per Lorenzo Stoakes and Shakeel Butt;
- Changed vma_clear() and vma_copy() to set required vma members
individually, per Matthew Wilcox
- Added comments in vma_start_read() about new false locked result
possibilities, per Vlastimil Babka
- Added back freeptr_t into vm_area_struct, as it's harmless and can be
used in the future to shrink the structure, per Vlastimil Babka
- Fixed the race in vm_area_alloc() when vma->detached was temporarily
reset by memcpy() before setting it back, per Vlastimil Babka
- Added a patch allowing freeptr_offset to be used with ctor,
per Vlastimil Babka
Patch applies over linux-next (due to vm_lock change [5] not in mm tree).
[1] https://lore.kernel.org/all/20230227173632.3292573-34-surenb@google.com/
[2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/
[3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@mail.gmail.com/
[4] https://lore.kernel.org/all/20241120000826.335387-1-surenb@google.com/
[5] https://lore.kernel.org/all/20241122174416.1367052-2-surenb@google.com/
Suren Baghdasaryan (6):
mm: introduce vma_start_read_locked{_nested} helpers
mm: move per-vma lock into vm_area_struct
mm: mark vma as detached until it's added into vma tree
mm: make vma cache SLAB_TYPESAFE_BY_RCU
mm/slab: allow freeptr_offset to be used with ctor
docs/mm: document latest changes to vm_lock
Documentation/mm/process_addrs.rst | 10 +-
include/linux/mm.h | 107 ++++++++++++++----
include/linux/mm_types.h | 16 ++-
include/linux/slab.h | 11 +-
kernel/fork.c | 168 ++++++++++++++++++++---------
mm/memory.c | 17 ++-
mm/slub.c | 2 +-
mm/userfaultfd.c | 22 +---
mm/vma.c | 8 +-
mm/vma.h | 2 +
tools/testing/vma/vma_internal.h | 55 ++++------
11 files changed, 267 insertions(+), 151 deletions(-)
base-commit: ebe1b11614e079c5e366ce9bd3c8f44ca0fbcc1b
--
2.47.0.338.g60cca15819-goog
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