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 <[email protected]>
---
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
