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

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

> On Jul 14, 2026, at 17:45, Muchun Song <muchun.song@linux.dev> wrote:
> 
> 
> 
> 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.

BTW, If you really want to properly support KASAN, you should integrate
the check directly into memcpy_flushcache() via kasan_check_write(), rather
than treating it as a special case here.

> 
> 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
>> 
>