memmap_init_zone_device() can take a noticeable amount of time when large pmem namespaces are bound or rebound, because it initializes nearly identical struct page descriptors one PFN at a time. This series reduces that ZONE_DEVICE memmap initialization overhead by reusing prepared struct page templates and, on x86, using memcpy_nontemporal() for the template copy path.
The main target is large fsdax/devdax pmem configurations, where the cost of initializing the memmap shows up directly in nd_pmem/dax_pmem bind and rebind latency. This matters because the cost is paid in the synchronous probe/bind path for large DAX/PMEM ZONE_DEVICE mappings. Userspace workflows such as provisioning or reconfiguring nd_pmem/dax_pmem namespaces, bringing hot-added PMEM-backed capacity online, and recovering or rebinding a device after driver or device changes all wait for this initialization to finish. Reducing this cost will yield benefits as lower user-visible provisioning, hot-add, recovery, and rebind latency for large DAX/PMEM devices. Patches 1-2 are preparatory cleanups and helper extraction. Patches 3-4 add the template-copy path for head pages and compound tails. Patch 5 introduces memcpy_nontemporal(). Patch 6 switches the ZONE_DEVICE template-copy path over to memcpy_nontemporal(). Patch 7 extends the x86 fixed-size memcpy_flushcache() inline cases used by the x86 memcpy_nontemporal() backend for struct page sized copies. Architectures without a specialized memcpy_nontemporal() backend fall back to memcpy(), so the generic template-copy optimization remains available without arch-specific support. On x86, memcpy_nontemporal() maps to the existing memcpy_flushcache() backend and can use the fixed-size MOVNTI paths added by this series for struct page sized copies. memcpy_nontemporal() is only a copy primitive. It does not imply a drain or a publication barrier. Callers that use it before a producer-consumer or device-visible handoff must provide the required ordering. The ZONE_DEVICE template-copy path uses it only while initializing struct page metadata, so the copy primitive itself does not grow a separate drain contract. The numbers below measure the time spent in memmap_init_zone_device() during driver bind/rebind. They are not measurements of the full nd_pmem or dax_pmem bind/rebind operation. Tested in an x86_64 QEMU/KVM 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 drivers 30 times and collect the memmap initialization time from the pr_debug() output of memmap_init_zone_device(). Base(v7.3-rc1): Average of nd_pmem rebinds: 221.07 ms Average of dax_pmem rebinds: 191.20 ms With this series applied: Average of nd_pmem rebinds: 71.93 ms Average of dax_pmem rebinds: 87.37 ms This reduces the average memmap initialization time measured during rebind by about 67.5% for nd_pmem and 54.3% for dax_pmem. As an additional x86_64 data point, I also ran measurements on the same physical host with a 100 GB PMEM region created via the memmap= kernel command line, configured as fsdax and devdax namespaces with map=dev and 2 MiB alignment. For brevity, the individual patches keep only the VM results rather than including a second set of physical-host measurements throughout the series. The physical-host numbers below are included only as supplemental evidence that the same optimization also provides a similar benefit on a non-virtualized system. Test procedure: Reconfigure the namespace mode, rebind the nd_pmem or dax_pmem driver 30 times, and collect the memmap initialization time from the pr_debug() output of memmap_init_zone_device(). Base (v7.3-rc1): nd_pmem / fsdax: 205.90 ms dax_pmem / devdax: 225.43 ms With this series applied: nd_pmem / fsdax: 69.13 ms dax_pmem / devdax: 90.67 ms This reduces the measured memmap initialization time during rebind by about 66.4% for nd_pmem and 59.8% for dax_pmem on that setup, which is broadly consistent with the VM results above. As another supplemental data point, I measured the test_hmm.ko module on the same physical x86_64 host, using the test_hmm.ko setup from the previous discussion that times ten 64 GB memremap_pages()/memunmap_pages() iterations during module insertion[1]. By default, module insertion initializes two DEVICE_PRIVATE dmirror devices, so two avg memremap values are reported; each value is the average for one 64 GB chunk. This is not the primary target workload of the series, but it exercises the same large ZONE_DEVICE memmap initialization path and shows the same direction of improvement. Base (v7.3-rc1): avg memremap reported during module insertion: 116500596 ns, 116438028 ns With this series applied: avg memremap reported during module insertion: 46953088 ns, 46428399 ns This corresponds to about a 59.9% reduction based on the mean of the reported values, which is again consistent with the pmem bind/rebind results above. I also include an arm64 data point for the generic template-copy part. It was measured on an arm64 QEMU virt VM with 64 KB pages and a 100 GB ACPI NVDIMM sparse backend. This setup does not use the x86 MOVNTI fast paths, so it exercises the architecture-independent part of the optimization. For devdax, 2 MiB alignment is rejected in this 64 KB page setup, so the devdax namespace was tested with the supported default 512 MiB alignment. Base (v7.3-rc1): Average of rebinds for nd_pmem driver: 27.93 ms Average of rebinds for dax_pmem driver: 27.87 ms With this series applied: Average of rebinds for nd_pmem driver: 14.53 ms Average of rebinds for dax_pmem driver: 16.27 ms This reduces the average memmap initialization time measured during rebind by about 48.0% for nd_pmem and 41.6% for dax_pmem on that arm64 VM setup. Since this arm64 setup does not use the x86 MOVNTI fast paths, the result also suggests that the generic template-copy optimization can benefit architectures without an architecture-specific memcpy_nontemporal() backend. [1] https://lore.kernel.org/all/[email protected]/ Li Zhe (7): mm: fix stale ZONE_DEVICE refcount comment mm: add a set_page_section_from_pfn() helper mm: add a template-based fast path for zone-device page init mm: extend the template fast path to zone-device compound tails string: introduce memcpy_nontemporal() mm: use memcpy_nontemporal() in zone-device template copies x86/string: extend memcpy_flushcache() fixed-size fastpaths arch/x86/include/asm/string_64.h | 83 ++++++++++++++++++++++++++------ include/linux/mm.h | 15 ++++-- include/linux/string.h | 13 +++++ mm/mm_init.c | 72 +++++++++++++++++++++------ 4 files changed, 149 insertions(+), 34 deletions(-) --- v10: https://lore.kernel.org/all/[email protected]/ v9: https://lore.kernel.org/all/[email protected]/ v8: https://lore.kernel.org/all/[email protected]/ v7: https://lore.kernel.org/all/[email protected]/ v6: https://lore.kernel.org/all/[email protected]/ v5: https://lore.kernel.org/all/[email protected]/ v4: https://lore.kernel.org/all/[email protected]/ v3: https://lore.kernel.org/all/[email protected]/ v2: https://lore.kernel.org/all/[email protected]/ v1: https://lore.kernel.org/all/[email protected]/ Changelogs: v10->v11: - Rebased the series on v7.3-rc1. - Refresh the benchmark numbers on v7.3-rc1. - Dropped the standalone helper split around __init_zone_device_page(); keep the existing helper shape and layer the template path directly on top. Suggested by Mike Rapoport. - Move the first head-page/tail-page initialization out of the template-copy loops. Suggested by Mike Rapoport. - Fold the template PFN-dependent field refresh into the template-copy helper instead of keeping a separate zone_device_page_update_template() helper. Suggested by Mike Rapoport. For changelogs of earlier revisions, please refer to the v10 cover letter. -- 2.20.1
