Re: [PATCH v6 4/8] mm: add a template-based fast path for zone-device page init
From: Li Zhe
Date: Wed Jul 15 2026 - 00:22:12 EST
On 7/13/26 9:28 PM, Muchun Song wrote:
>
>
> On 2026/7/9 19:25, Li Zhe wrote:
>> memmap_init_zone_device() repeats nearly identical head-page
>> initialization for each PFN. Prepare one reusable ZONE_DEVICE head-page
>> template through the existing slow path, refresh the PFN-dependent
>> fields in that template before each copy, and memcpy it into each
>> destination page.
>>
>> The optimized path assigns _refcount through the copied template, so
>> keep it disabled when the page_ref_set tracepoint is enabled.
>>
>> 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(v7.2-rc1):
>> First binding: 1456 ms
>> Average of subsequent rebinds: 244.28 ms
>>
>> With this patch and its prerequisites applied:
>> First binding: 1440 ms
>> Average of subsequent rebinds: 217.19 ms
>>
>> This reduces the average rebind time from 244.28 ms to 217.19 ms, or
>> about 11%.
>
> An 11% reduction is definitely a solid achievement. While the
> absolute gain is in the range of a few dozen milliseconds, I’d
> love to better understand the compounding impact. In which
> specific latency-sensitive scenarios or workloads does saving
> these few dozen milliseconds translate into a major breakthrough
> for the user experience?
>
> Muchun,
> Thanks.
Patch 4 by itself is only an incremental step, not the full result of
the series.
The target scenario here is large fsdax/devdax pmem bind/rebind
latency. This patch removes the repeated head-page initialization
overhead on the pfns_per_compound == 1 path, so the gain at this stage
is only a few dozen milliseconds.
The later patches extend the same idea to compound tails and then switch
the template-copy path over to memcpy_nt(), which is where the larger
reduction reported for the full series comes from. Even this
incremental reduction still helps shorten the control-path latency for
large namespace bind/rebind operations.
Thanks,
Zhe
>
>>
>> Signed-off-by: Li Zhe <lizhe.67@xxxxxxxxxxxxx>
>> ---
>> mm/mm_init.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>> 1 file changed, 58 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>> index 5fccfbacf855..170021e182e0 100644
>> --- a/mm/mm_init.c
>> +++ b/mm/mm_init.c
>> @@ -1065,6 +1065,44 @@ static void __ref
>> zone_device_page_init_slow(struct page *page,
>> set_page_count(page, 0);
>> }
>> +static inline bool zone_device_page_init_optimization_enabled(void)
>> +{
>> + /*
>> + * The template fast path copies a preinitialized struct page
>> image.
>> + * Skip it when the page_ref_set tracepoint is enabled.
>> + */
>> + return !page_ref_tracepoint_active(page_ref_set);
>> +}
>> +
>> +/*
>> + * 'template' is a reusable page prototype rather than a strictly
>> immutable
>> + * object. Most ZONE_DEVICE fields stay constant across the pages
>> covered by
>> + * the current template, but section bits and page->virtual may
>> still depend
>> + * on the PFN. Refresh those PFN-dependent fields in the template
>> before
>> + * copying it into @page.
>> + */
>> +static inline void zone_device_page_update_template(struct page
>> *template,
>> + unsigned long pfn)
>> +{
>> + 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
>> +}
>> +
>> +static void zone_device_page_init_from_template(struct page *page,
>> + unsigned long pfn, struct page *template)
>> +{
>> + /*
>> + * 'template' carries the invariant portion of a ZONE_DEVICE struct
>> + * page. Update the PFN-dependent fields in place before copying it
>> + * to the destination page.
>> + */
>> + zone_device_page_update_template(template, pfn);
>> + 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
>> @@ -1120,6 +1158,7 @@ void __ref memmap_init_zone_device(struct zone
>> *zone,
>> unsigned long nr_pages,
>> struct dev_pagemap *pgmap)
>> {
>> + bool use_template = zone_device_page_init_optimization_enabled();
>> unsigned long pfn, end_pfn = start_pfn + nr_pages;
>> struct pglist_data *pgdat = zone->zone_pgdat;
>> struct vmem_altmap *altmap = pgmap_altmap(pgmap);
>> @@ -1127,6 +1166,7 @@ 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;
>> if (WARN_ON_ONCE(!pgmap || zone_idx != ZONE_DEVICE))
>> return;
>> @@ -1144,7 +1184,24 @@ void __ref memmap_init_zone_device(struct zone
>> *zone,
>> for (pfn = start_pfn; pfn < end_pfn; pfn += pfns_per_compound) {
>> struct page *page = pfn_to_page(pfn);
>> - zone_device_page_init_slow(page, pfn, zone_idx, nid, pgmap);
>> + if (!use_template) {
>> + zone_device_page_init_slow(page, pfn, zone_idx,
>> + nid, pgmap);
>> + } else if (pfn == start_pfn) {
>> + /*
>> + * Seed the reusable head-page template from the
>> + * first real struct page, because the existing
>> + * page-init and pageblock helpers expect a real
>> + * memmap entry rather than a stack object.
>> + */
>> + zone_device_page_init_slow(page, pfn, zone_idx,
>> + nid, pgmap);
>> + /* init template page */
>> + memcpy(&template, page, sizeof(*page));
>> + } else {
>> + zone_device_page_init_from_template(page, pfn,
>> + &template);
>> + }
>>
>> if (IS_ALIGNED(pfn, PAGES_PER_SECTION))
>> cond_resched();
>> --
>> 2.20.1
>