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

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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>
This commit is contained in:
Li Zhe 2026-08-31 19:16:34 +08:00 committed by Andrew Morton
parent a5d43b7cd6
commit 72dcfdc2ce
1 changed files with 34 additions and 3 deletions
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37
mm/mm_init.c
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@ -1029,6 +1029,17 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
}
}
static void zone_device_page_init_from_template(struct page *page,
unsigned long pfn, struct page *template)
{
set_page_section_from_pfn(template, pfn);
#ifdef WANT_PAGE_VIRTUAL
if (!is_highmem_idx(ZONE_DEVICE))
set_page_address(template, __va(pfn << PAGE_SHIFT));
#endif
memcpy(page, template, sizeof(*page));
}
/*
* With compound page geometry and when struct pages are stored in ram most
* tail pages are reused. Consequently, the amount of unique struct pages to
@ -1091,6 +1102,8 @@ void __ref memmap_init_zone_device(struct zone *zone,
unsigned long zone_idx = zone_idx(zone);
unsigned long start = jiffies;
int nid = pgdat->node_id;
struct page template;
struct page *page;
if (WARN_ON_ONCE(!pgmap || zone_idx != ZONE_DEVICE))
return;
@ -1105,10 +1118,28 @@ void __ref memmap_init_zone_device(struct zone *zone,
nr_pages = end_pfn - start_pfn;
}
for (pfn = start_pfn; pfn < end_pfn; pfn += pfns_per_compound) {
struct page *page = pfn_to_page(pfn);
if (!nr_pages)
return;
__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
/*
* Seed the reusable head-page template from the first real struct
* page. The normal page-init and refcount helpers must operate on
* a real memmap entry rather than a stack object.
*/
pfn = start_pfn;
page = pfn_to_page(pfn);
__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
memcpy(&template, page, sizeof(*page));
if (pfns_per_compound != 1)
memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
compound_nr_pages(pfn, altmap, pgmap));
pfn += pfns_per_compound;
/* Initialize the remaining head pages from template. */
for (; pfn < end_pfn; pfn += pfns_per_compound) {
page = pfn_to_page(pfn);
zone_device_page_init_from_template(page, pfn, &template);
if (IS_ALIGNED(pfn, PAGES_PER_SECTION))
cond_resched();