[PATCH v6 0/8] mm: optimize zone-device memmap initialization

memmap_init_zone_device() can take a noticeable amount of time when large
pmem namespaces are bound or rebound, because it initializes nearly
identical struct page descriptors one PFN at a time. This series reduces
that ZONE_DEVICE memmap initialization overhead by reusing prepared
struct page templates and, on x86, using memcpy_nt() for the template
copy path.

The main target is large fsdax/devdax pmem configurations, where the
cost of initializing the memmap shows up directly in nd_pmem/dax_pmem
bind and rebind latency.

Patches 1-3 are preparatory cleanups and helper extraction. Patches 4-5
add the template-copy fast path for head pages and compound tails.
Patches 6-8 introduce memcpy_nt()/memcpy_nt_drain(), extend the x86
fixed-size memcpy_flushcache() inline cases used by that helper, and
switch the template-copy path over to memcpy_nt().

The fast path remains disabled when the page_ref_set tracepoint is
active, and sanitized builds stay on the slow path so their instrumented
stores are preserved. Architectures without a specialized memcpy_nt()
backend continue to fall back to memcpy().

Tested in a VM with a 100 GB fsdax namespace device configured with
map=dev and a 100 GB devdax namespace (align=2097152) on Intel Ice Lake
server.

Test procedure:
Rebind the nd_pmem and dax_pmem driver 30 times and collect the memmap
initialization time from the pr_debug() output of
memmap_init_zone_device().

Base(v7.2-rc1):
  First binding for nd_pmem driver: 1456 ms
  Average of subsequent rebinds: 244.28 ms

  First binding for dax_pmem driver: 1462 ms
  Average of subsequent rebinds: 273.31 ms

With this series applied:
  First binding for nd_pmem driver: 1272 ms
  Average of subsequent rebinds: 96.79 ms

  First binding for dax_pmem driver: 1354 ms
  Average of subsequent rebinds: 119.04 ms

This reduces the average rebind time by about 60.4% for nd_pmem and
56.4% for dax_pmem.

As an additional data point, I also ran a smaller set of measurements on
the same physical x86_64 host with a 100 GB PMEM region created via the
memmap= kernel command line, configured as fsdax and devdax namespaces
with map=dev and 2 MiB alignment.

For brevity, the individual patches keep only the VM results rather than
including a second set of physical-host measurements throughout the
series. The physical-host numbers below are included only as
supplemental evidence that the same optimization also provides a similar
benefit on a non-virtualized system.

Test procedure:
Reconfigure the namespace mode, rebind the nd_pmem or dax_pmem driver
once, and collect the memmap initialization time from the pr_debug()
output of memmap_init_zone_device().

Base (v7.2-rc1):
  nd_pmem / fsdax: 179 ms
  dax_pmem / devdax: 264 ms

With this series applied:
  nd_pmem / fsdax: 82 ms
  dax_pmem / devdax: 113 ms

This reduces the measured rebind time by about 54.2% for nd_pmem and
57.2% for dax_pmem on that setup, which is broadly consistent with the
VM results above.

As another supplemental data point, I also measured the test_hmm.ko
module on the same physical x86_64 host, using the test_hmm.ko setup
from the previous discussion that times ten 64 GB
memremap_pages()/memunmap_pages() iterations during module insertion[1].
By default, module insertion initializes two DEVICE_PRIVATE dmirror
devices, so two avg memremap values are reported; each value is the
average for one 64 GB chunk.

This is not the primary target workload of the series, but it exercises
the same large ZONE_DEVICE memmap initialization path and shows the same
direction of improvement.

Base (v7.2-rc1):
  avg memremap reported during module insertion: 116689362 ns, 116539263 ns

With this series applied:
  avg memremap reported during module insertion: 54607108 ns, 54458236 ns

This corresponds to about a 53.2% reduction based on the mean of the
reported values, which is again consistent with the pmem bind/rebind
results above.

[1] https://lore.kernel.org/all/[email protected]/

Li Zhe (8):
  mm: fix stale ZONE_DEVICE refcount comment
  mm: factor zone-device page init helpers out of
    __init_zone_device_page
  mm: add a set_page_section_from_pfn() helper
  mm: add a template-based fast path for zone-device page init
  mm: extend the template fast path to zone-device compound tails
  string: introduce memcpy_nt() helpers
  x86/string: extend memcpy_flushcache() fixed-size fastpaths
  mm: use memcpy_nt() in zone-device template copies

 arch/x86/include/asm/string_64.h |  78 ++++++++++++-
 include/linux/mm.h               |  15 ++-
 include/linux/string.h           |  23 ++++
 mm/mm_init.c                     | 186 ++++++++++++++++++++++++++-----
 4 files changed, 273 insertions(+), 29 deletions(-)

---
v5: https://lore.kernel.org/all/[email protected]/
v4: https://lore.kernel.org/all/[email protected]/
v3: https://lore.kernel.org/all/[email protected]/
v2: https://lore.kernel.org/all/[email protected]/
v1: https://lore.kernel.org/all/[email protected]/

Changelogs:

v5->v6:
- Rework patch 6 so the x86 memcpy_nt_drain() helper uses wmb()
  instead of an open-coded sfence, while architectures that do not
  override memcpy_nt() keep the generic no-op drain fallback.
  Suggested by Borislav Petkov.
- Rework patch 6 to use the usual self-macro override pattern for
  memcpy_nt() and memcpy_nt_drain() instead of a dedicated
  __HAVE_ARCH_MEMCPY_NT feature macro. Suggested by David
  Hildenbrand.
- Drop the default: case from pagemap_resets_refcount() and drop the
  WARN_ONCE() after the switch so newly added enum memory_type values
  remain visible to compiler switch checking. Suggested by David
  Hildenbrand.
- Drop the unnecessary empty set_page_section() stub from patch 3, keep
  set_page_section_from_pfn() as the only !SECTION_IN_PAGE_FLAGS no-op
  helper and fix its indentation. Suggested by David Hildenbrand.
- Rework patch 4 so the reusable head-page template is seeded from the
  first loop iteration, keeping the main loop intact while ensuring the
  first page is initialized only once; also drop the extra template
  memcpy helper, as suggested by Alistair Popple.
- Rework patch 5 similarly for compound tails: seed the reusable tail
  template from the first tail-page iteration so the first tail page is
  initialized only once, while keeping the main tail loop intact, as
  suggested by Alistair Popple.
- Rework patch 7 to rename the internal fixed-size MOVNTI helpers to
  movnti_*(), keep memcpy_flushcache() as the externally visible
  wrapper, fold the new 32/48/64/80/96-byte copies into the main
  fixed-size switch, clarify in the code and changelog that the
  ZONE_DEVICE template-copy path relies on the 64/80/96-byte struct
  page copies while 32/48-byte copies stay inline as well, and drop the
  separate alignment check based on the physical-host microbenchmark
  results. Suggested by Borislav Petkov.
- Run v6 VM spot checks for the fsdax/devdax map=dev paths. The
  results stayed in the same ballpark as v5, so keep the previously
  posted v5 full-run numbers in this version instead of replacing them
  with another partial data set.

For changelogs of earlier revisions, please refer to the v5 cover letter.

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2.20.1