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From: Muchun Song In-Reply-To: Date: Wed, 15 Jul 2026 17:45:15 +0800 Cc: linux-arch@vger.kernel.org, linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, apopple@nvidia.com, arnd@arndb.de, balbirs@nvidia.com, bp@alien8.de, dave.hansen@linux.intel.com, david@kernel.org, kees@kernel.org, mingo@redhat.com, rppt@kernel.org, tglx@kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: <10E87709-2116-46E3-96E2-6BC4E6F69952@linux.dev> References: <20260709112520.24857-1-lizhe.67@bytedance.com> <20260709112520.24857-9-lizhe.67@bytedance.com> <04f5b50a-a526-4644-b9f0-c54a0d6db702@linux.dev> <67BC9BE5-A201-4DED-B847-8C74BF30B16D@linux.dev> To: Li Zhe X-Migadu-Flow: FLOW_OUT > On Jul 15, 2026, at 14:30, Li Zhe wrote: >=20 > On 7/14/26 7:22 PM, Muchun Song wrote: >>=20 >>> On Jul 14, 2026, at 17:45, Muchun Song = wrote: >>>=20 >>>=20 >>>=20 >>> 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(). >>>>=20 >>>> 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(). >>>>=20 >>>> 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(). >>>>=20 >>>> Keep sanitized builds on the slow path so KASAN/KMSAN retain their >>>> instrumented stores. >>>>=20 >>>> Tested in a VM with a 100 GB fsdax namespace device configured with >>>> map=3Ddev and a 100 GB devdax namespace (align=3D2097152) on Intel = Ice Lake >>>> server. >>>>=20 >>>> 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(). >>>>=20 >>>> Base(v7.2-rc1): >>>> First binding for nd_pmem driver: 1456 ms >>>> Average of subsequent rebinds: 244.28 ms >>>>=20 >>>> First binding for dax_pmem driver: 1462 ms >>>> Average of subsequent rebinds: 273.31 ms >>>>=20 >>>> With this series: >>>> First binding for nd_pmem driver: 1272 ms >>>> Average of subsequent rebinds: 96.79 ms >>>>=20 >>>> First binding for dax_pmem driver: 1354 ms >>>> Average of subsequent rebinds: 119.04 ms >>>>=20 >>>> This reduces the average rebind time by about 60.4% for nd_pmem and >>>> 56.4% for dax_pmem. >>>>=20 >>>> Signed-off-by: Li Zhe >>>> --- >>>> mm/mm_init.c | 39 +++++++++++++++++++++++++++++++++++++-- >>>> 1 file changed, 37 insertions(+), 2 deletions(-) >>>>=20 >>>> 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, >>>>=20 >>>> 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. >=20 > 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. >=20 > 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. >=20 > Thanks, > Zhe >=20 >>=20 >>> Thanks. >>>=20 >>>> + >>>> /* >>>> * 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; >>>> } >>>>=20 >>>> 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)); >>>> } >>>>=20 >>>> /* >>>> @@ -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); >>>> } >>>>=20 >>>> @@ -1238,10 +1257,26 @@ void __ref memmap_init_zone_device(struct = zone *zone, >>>> if (pfns_per_compound =3D=3D 1) >>>> continue; >>>>=20 >>>> + /* >>>> + * 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(); >>>>=20 >>>> pageblock_migratetype_init_range(start_pfn, nr_pages, = MIGRATE_MOVABLE); >>>>=20 >>>> -- >>>> 2.20.1