mm: add a template-based fast path for zone-device page init

memmap_init_zone_device() repeats nearly identical head-page
initialization for each PFN.  Initialize the first real ZONE_DEVICE head
page through the existing path, copy that final state into a reusable
template, refresh the PFN-dependent fields in that template before each
copy, and copy it into the remaining destination pages.

Use the template path unconditionally.  The page_ref_set tracepoint is
primarily a debugging aid, while this code is still initializing struct
pages before they are handed out.  From the perspective of users of those
pages, the initialization-time refcount transitions are not part of the
observable page lifetime.

This means page_ref_set will no longer observe every initialization-time
refcount assignment for copied ZONE_DEVICE head pages.  The impact is
controlled because the final initialized struct page state is unchanged,
and keeping a separate non-template path only for this local tracepoint
observability would add complexity to the common path.

This patch accelerates head-page initialization.  The pfns_per_compound ==
1 case gets the full benefit here, compound tails are handled in the next
patch.

Tested in a VM with a 100 GB fsdax namespace device configured with
map=dev on Intel Ice Lake server.  This test exercises the nd_pmem rebind
path (pfns_per_compound == 1).

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

Base(v7.3-rc1):
  Average of rebinds for nd_pmem driver: 221.07 ms

With this patch and its prerequisites applied:
  Average of rebinds for nd_pmem driver: 155.00 ms

This reduces the average memmap initialization time measured during
rebind from 221.07 ms to 155.00 ms, or about 29.9%.

[lizhe.67@bytedance.com: whitespace fix, per Mike]
  Link: https://lore.kernel.org/20260903025806.70825-1-lizhe.67@bytedance.com
Link: https://lore.kernel.org/20260831111638.76012-4-lizhe.67@bytedance.com
Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: "Borislav Petkov (AMD)" <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>

0 files changed, 0 insertions, 0 deletions