On 7/13/26 9:15 PM, Muchun Song wrote: > >> On Jul 9, 2026, at 19:25, Li Zhe <[email protected]> wrote: >> >> 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 > Hi, I have a detailed question regarding the timing of the bound or rebound > process. > > 1. How long does the entire process take? > 2. Specifically, how long does the struct page initialization take? > > I'm trying to understand the time distribution and what percentage of the > total process is occupied by the struct page initialization. What I measured and reported in this version is the memmap_init_zone_device() time from its pr_debug() output.
Separately, on my setup, a flamegraph of the pmem driver bind path showed __init_zone_device_page() dominating the operation and accounting for more than 90% of the module insertion time, which is why this series focuses on that path. >> 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. > Regarding the 60% reduction in time: does this refer to the entire process, > or specifically to the initialization of struct pages in > memmap_init_zone_device()? > > Thanks. It refers specifically to the measured memmap_init_zone_device() time, not to the full bind/rebind wall time. Thanks, Zhe >> 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 | 78 ++++++++++++- >> include/linux/mm.h | 15 ++- >> include/linux/string.h | 23 ++++ >> mm/mm_init.c | 186 ++++++++++++++++++++++++++----- >> 4 files changed, 273 insertions(+), 29 deletions(-) >> >> --- >> 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: >> >> v5->v6: >> - Rework patch 6 so the x86 memcpy_nt_drain() helper uses wmb() >> instead of an open-coded sfence, while architectures that do not >> override memcpy_nt() keep the generic no-op drain fallback. >> Suggested by Borislav Petkov. >> - Rework patch 6 to use the usual self-macro override pattern for >> memcpy_nt() and memcpy_nt_drain() instead of a dedicated >> __HAVE_ARCH_MEMCPY_NT feature macro. Suggested by David >> Hildenbrand. >> - Drop the default: case from pagemap_resets_refcount() and drop the >> WARN_ONCE() after the switch so newly added enum memory_type values >> remain visible to compiler switch checking. Suggested by David >> Hildenbrand. >> - Drop the unnecessary empty set_page_section() stub from patch 3, keep >> set_page_section_from_pfn() as the only !SECTION_IN_PAGE_FLAGS no-op >> helper and fix its indentation. Suggested by David Hildenbrand. >> - Rework patch 4 so the reusable head-page template is seeded from the >> first loop iteration, keeping the main loop intact while ensuring the >> first page is initialized only once; also drop the extra template >> memcpy helper, as suggested by Alistair Popple. >> - Rework patch 5 similarly for compound tails: seed the reusable tail >> template from the first tail-page iteration so the first tail page is >> initialized only once, while keeping the main tail loop intact, as >> suggested by Alistair Popple. >> - Rework patch 7 to rename the internal fixed-size MOVNTI helpers to >> movnti_*(), keep memcpy_flushcache() as the externally visible >> wrapper, fold the new 32/48/64/80/96-byte copies into the main >> fixed-size switch, clarify in the code and changelog that the >> ZONE_DEVICE template-copy path relies on the 64/80/96-byte struct >> page copies while 32/48-byte copies stay inline as well, and drop the >> separate alignment check based on the physical-host microbenchmark >> results. Suggested by Borislav Petkov. >> - Run v6 VM spot checks for the fsdax/devdax map=dev paths. The >> results stayed in the same ballpark as v5, so keep the previously >> posted v5 full-run numbers in this version instead of replacing them >> with another partial data set. >> >> For changelogs of earlier revisions, please refer to the v5 cover letter. >> >> -- >> 2.20.1
