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_nt() 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. Patches 1-3 are preparatory cleanups and helper extraction. Patches 4-5 add the template-copy fast path for head pages and compound tails. Patches 6-8 introduce memcpy_nt()/memcpy_nt_drain(), extend the x86 fixed-size memcpy_flushcache() inline cases used by that helper, and switch the template-copy path over to memcpy_nt(). The fast path remains disabled when the page_ref_set tracepoint is active, and sanitized builds stay on the slow path so their instrumented stores are preserved. Architectures without a specialized memcpy_nt() backend continue to fall back to memcpy(). 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 applied: 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. As an additional data point, I also ran a smaller set of measurements on the same physical x86_64 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 once, and collect the memmap initialization time from the pr_debug() output of memmap_init_zone_device(). Base (v7.2-rc1): nd_pmem / fsdax: 179 ms dax_pmem / devdax: 264 ms With this series applied: nd_pmem / fsdax: 82 ms dax_pmem / devdax: 113 ms This reduces the measured rebind time by about 54.2% for nd_pmem and 57.2% for dax_pmem on that setup, which is broadly consistent with the VM results above. As another supplemental data point, I also 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 (v7.2-rc1): avg memremap reported during module insertion: 116689362 ns, 116539263 ns With this series applied: avg memremap reported during module insertion: 54607108 ns, 54458236 ns This corresponds to about a 53.2% reduction based on the mean of the reported values, which is again consistent with the pmem bind/rebind results above. [1] https://lore.kernel.org/all/[email protected]/ Li Zhe (8): mm: fix stale ZONE_DEVICE refcount comment mm: factor zone-device page init helpers out of __init_zone_device_page 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_nt() helpers x86/string: extend memcpy_flushcache() fixed-size fastpaths mm: use memcpy_nt() in zone-device template copies arch/x86/include/asm/string_64.h | 96 +++++++++++++- include/linux/mm.h | 19 ++- include/linux/string.h | 18 +++ mm/mm_init.c | 209 +++++++++++++++++++++++++++---- 4 files changed, 311 insertions(+), 31 deletions(-) --- 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: v4->v5: - Rebase the series from v7.1-rc6 to v7.2-rc1, and refresh the VM performance numbers. - Simplify patch 6 around a small memcpy_nt()/memcpy_nt_drain() interface, rename the previous memcpy_streaming() helpers accordingly, make the generic implementation fall back to memcpy(), and let x86 reuse the existing memcpy_flushcache() backend instead of carrying extra policy/alignment logic in the generic layer. Suggested by Borislav Petkov. - Add physical-host measurements for a 100 GB PMEM region simulated via the memmap= kernel command line to the cover letter as supplemental evidence that the same optimization also improves fsdax/devdax map=dev bind/rebind latency outside the VM, while keeping the per-patch performance data limited to the VM measurements for consistency across the series. Suggested by Borislav Petkov. - Add supplemental test_hmm.ko results to the cover letter as another physical-host data point, in addition to the pmem bind/rebind measurements. v3->v4: - Rebase the series from v7.1-rc3 to v7.1-rc6. - Rework patch 4 so the reusable head-page template is seeded from the first real struct page, rather than being initialized directly on a stack-resident template object. Also add an explicit !nr_pages early return. Suggested by Andrew Morton. - Rework patch 5 similarly for compound tails: seed the reusable tail-page template from the first real tail page, thread use_template through compound-page initialization, and reuse that prepared tail-page image for the remaining tails. Suggested by Andrew Morton. - Tighten patch 6 so memcpy_streaming() maps to memcpy_flushcache() only when the destination alignment and size allow the transfer to stay entirely on the non-temporal path; other cases fall back to memcpy(). Suggested by Andrew Morton. - Rework patch 7 so the existing 4/8/16-byte cases remain handled directly in memcpy_flushcache(), while the new aligned fixed-size fastpaths cover only the larger 32/48/64/80/96-byte cases. Suggested by Andrew Morton. For changelogs of earlier revisions, please refer to the v3 cover letter: https://lore.kernel.org/all/[email protected]/ -- 2.20.1
