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

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

> On Jul 15, 2026, at 14:30, Li Zhe <lizhe.67@bytedance.com> wrote:
> 
> On 7/14/26 7:22 PM, Muchun Song wrote:
>> 
>>> 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.
> I agree that proper KASAN-aware support, if ever needed for
> memcpy_flushcache(), would belong in the low-level helper itself rather
> than in this caller.
> 
> That said, for this series I would still prefer to keep the local check
> in zone_device_page_init_optimization_enabled(). The goal here is not to
> add full KASAN support to memcpy_flushcache(), but simply to keep
> sanitized builds on the existing instrumented slow path.
> 
> When KASAN/KMSAN is enabled, performance is usually not the priority for
> that configuration, while preserving the instrumented store path is.
> Keeping the check local here avoids broadening the scope of this series
> into a generic memcpy_flushcache() change.

However, there is an even simpler approach: just don't do any special handling
for KASAN. In fact, many callers of memcpy_flushcache don't handle it at all.

So I'm curious, why are you keen on handling KASAN as a special case here?

Thanks.

> 
> Thanks,
> Zhe
> 
>> 
>>> 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