[PATCH v9 7/8] mm: use memcpy_nontemporal() in zone-device template copies

["Li Zhe" <lizhe.67@bytedance.com>]

The template fast path currently uses memcpy() for the actual struct
page copy. Switch zone_device_page_init_from_template() to
memcpy_nontemporal().

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_nontemporal()
lets this path request nontemporal 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().

Do not add a KASAN/KMSAN-specific fallback around this call site. As
Muchun pointed out, special KASAN handling for memcpy_flushcache() or
memcpy_nontemporal(), if needed, belongs in the low-level helper rather
than in this ZONE_DEVICE caller.

No separate drain is added here. memcpy_nontemporal() is used only as
the copy primitive while memmap_init_zone_device() is still initializing
the struct page array. The ordinary stores that follow in this path,
such as compound-page setup, are part of the same CPU's initialization
sequence; they are not used as a publication store that tells another CPU
or device to consume data written by the non-temporal copy.

Therefore this call site does not need a helper-level drain for
correctness. Callers that use memcpy_nontemporal() as part of a
producer-consumer or device-visible handoff must add the required
ordering themselves.

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):
  Average of rebinds for nd_pmem driver: 244.28 ms
  Average of rebinds for dax_pmem driver: 273.31 ms

With this patch and its prerequisites applied:
  Average of rebinds for nd_pmem driver: 150.83 ms
  Average of rebinds for dax_pmem driver: 153.55 ms

This reduces the average memmap initialization time measured during rebind
by about 38.3% for nd_pmem and 43.8% for dax_pmem.

Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
---
 mm/mm_init.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 9691fa2a060d..bb2007806a28 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1101,7 +1101,7 @@ 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_nontemporal(page, template, sizeof(*page));
 }
 
 /*
-- 
2.20.1