Re: [PATCH v6 8/8] mm: use memcpy_nt() in zone-device template copies

[Muchun Song <muchun.song@linux.dev>]

On 2026/7/9 19:25, Li Zhe wrote:
> The template fast path currently uses memcpy() for the actual struct
> page copy. Switch zone_device_page_init_from_template() to memcpy_nt()
> and add memcpy_nt_drain() before memmap_init_compound(), before
> prep_compound_head() updates overlapping tail metadata, and before
> returning from memmap_init_zone_device().
>
> ZONE_DEVICE memmap initialization is largely write-once: each struct
> page is populated once, and most destination cachelines are not expected
> to be reused immediately afterwards. On x86, a regular cached memcpy()
> can therefore incur write-allocate traffic by pulling destination
> cachelines into the cache before writeback, and can populate the cache
> with data that has little near-term reuse. Using memcpy_nt() lets this
> path request non-temporal stores for that copy pattern, which can reduce
> cache pollution and avoid part of the associated write-allocate
> overhead, while architectures without a specialized backend still fall
> back to memcpy().
>
> When memcpy_nt() maps to non-temporal stores, order those stores before
> memmap_init_compound(), before prep_compound_head() updates overlapping
> compound metadata, and before returning from memmap_init_zone_device().
>
> Keep sanitized builds on the slow path so KASAN/KMSAN retain their
> instrumented stores.
>
> 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:
>    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.
>
> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
> ---
>   mm/mm_init.c | 39 +++++++++++++++++++++++++++++++++++++--
>   1 file changed, 37 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index fb855bb0437a..addb4969587e 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1067,11 +1067,21 @@ static void __ref zone_device_page_init_slow(struct page *page,
>
>   static inline bool zone_device_page_init_optimization_enabled(void)
>   {
> +	/*
> +	 * Keep sanitized builds on the slow path so their stores stay
> +	 * instrumented.
> +	 */
> +	if (IS_ENABLED(CONFIG_KASAN) || IS_ENABLED(CONFIG_KMSAN))
> +		return false;

It is not a major concern if struct page initialization lacks KASAN or
KMSAN instrumentation. To keep things simple, let's just entirely remove
zone_device_page_init_optimization_enabled() to simplify the code.

Thanks.

> +
>   	/*
>   	 * 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);
> +	if (page_ref_tracepoint_active(page_ref_set))
> +		return false;
> +
> +	return true;
>   }
>
>   static inline void zone_device_tail_page_init(struct page *page,
> @@ -1110,7 +1120,7 @@ static void zone_device_page_init_from_template(struct page *page,
>   	 * to the destination page.
>   	 */
>   	zone_device_page_update_template(template, pfn);
> -	memcpy(page, template, sizeof(*page));
> +	memcpy_nt(page, template, sizeof(*page));
>   }
>
>   /*
> @@ -1179,6 +1189,15 @@ static void __ref memmap_init_compound(struct page *head,
>   							    &template);
>   		}
>   	}
> +
> +	/*
> +	 * When the template path is enabled, order the preceding tail-page copies
> +	 * before prep_compound_head() updates the overlapping compound metadata
> +	 * in the first tail-page descriptors. If memcpy_nt() fell back to
> +	 * regular cached stores, memcpy_nt_drain() may be a no-op.
> +	 */
> +	if (use_template)
> +		memcpy_nt_drain();
>   	prep_compound_head(head, order);
>   }
>
> @@ -1238,10 +1257,26 @@ void __ref memmap_init_zone_device(struct zone *zone,
>   		if (pfns_per_compound == 1)
>   			continue;
>
> +		/*
> +		 * When the template path is enabled, order the preceding head-page copy
> +		 * before memmap_init_compound(), which immediately updates compound-head
> +		 * metadata. If memcpy_nt() fell back to regular cached stores,
> +		 * memcpy_nt_drain() may be a no-op.
> +		 */
> +		if (use_template)
> +			memcpy_nt_drain();
> +
>   		memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
>   				     compound_nr_pages(pfn, altmap, pgmap),
>   				     use_template);
>   	}
> +	/*
> +	 * Ensure any prior template copies are ordered before returning.
> +	 * On architectures where memcpy_nt() used regular cached stores,
> +	 * memcpy_nt_drain() may be a no-op.
> +	 */
> +	if (use_template)
> +		memcpy_nt_drain();
>
>   	pageblock_migratetype_init_range(start_pfn, nr_pages, MIGRATE_MOVABLE);
>
> --
> 2.20.1
>