On Thu, 4 Jun 2026 18:14:05 +1000, [email protected] wrote: > On 2026-06-03 at 18:01 +1000, Li Zhe <[email protected]> wrote... > > memmap_init_zone_device() can spend a substantial amount of time > > initializing large ZONE_DEVICE ranges because it repeats nearly > > identical struct page setup for every PFN. > > > > This series reduces that overhead in eight steps. > > > > The first patch fixes a stale comment in __init_zone_device_page() so > > the documented refcount policy matches the current ZONE_DEVICE code. > > > > The second patch factors the reusable pieces out of > > __init_zone_device_page() so later patches can share the same logic > > without changing the existing slow path. > > > > The third patch adds set_page_section_from_pfn(), so callers that want > > to refresh section bits from a PFN no longer need to open-code > > SECTION_IN_PAGE_FLAGS handling. > > > > The fourth patch adds a template-based fast path for ZONE_DEVICE head > > pages. Instead of rebuilding the same struct page state for every PFN, > > it prepares one reusable template through the existing slow path, > > refreshes the PFN-dependent fields in that template, and copies it to > > each destination page. > > > > The fifth patch extends the same template-based approach to compound > > tails, so pfns_per_compound > 1 can also benefit from the fast path. > > > > The sixth patch introduces memcpy_streaming() and > > memcpy_streaming_drain() as a generic interface for write-once copies. > > Architectures that do not provide a specialized backend, or cases that > > cannot safely use one, fall back to memcpy(). > > > > The seventh patch extends x86 memcpy_flushcache() small fixed-size > > fastpaths so struct-page-sized streaming copies can stay on the inline > > path when alignment permits. > > > > The last patch switches the ZONE_DEVICE template-copy path over to > > memcpy_streaming(). It keeps pageblock-aligned PFNs on regular memcpy(), > > uses memcpy_streaming() for the remaining write-once copies, and drains > > streaming stores before later metadata updates that may depend on them. > > > > This is not intended as a steady-state data-path optimization. Its > > benefit is in pmem bring-up paths where memmap_init_zone_device() > > dominates device online / rebind latency, such as: > > - fsdax or devdax namespace creation and reconfiguration > > - nd_pmem / dax_pmem driver bind or rebind > > > > In those paths, the kernel initializes a large vmemmap range once and > > does not immediately benefit from keeping the copied struct page state > > hot in cache. Reducing write-allocate traffic in that one-time setup > > path can therefore reduce end-to-end device bring-up latency. > > > > The optimized path is disabled when the page_ref_set tracepoint is > > enabled, and sanitized builds remain on the slow path so their > > instrumented stores are preserved. > > > > Testing > > ======= > > > > Tests were run in a VM on an Intel Ice Lake server. > > > > Two PMEM configurations were used: > > - a 100 GB fsdax namespace configured with map=dev, which exercises > > the nd_pmem rebind path (pfns_per_compound == 1) > > - a 100 GB devdax namespace configured with align=2097152, which > > exercises the dax_pmem rebind path (pfns_per_compound > 1) > > > > For each configuration, the corresponding driver was unbound and > > rebound 30 times. Memmap initialization latency was collected from the > > pr_debug() output of memmap_init_zone_device(). > > > > The first bind is reported separately, and the average of subsequent > > rebinds is used as the steady-state result. > > > > Performance > > =========== > > > > nd_pmem rebind, 100 GB fsdax namespace, map=dev > > Base(v7.1-rc6): > > First binding: 1466 ms > > Average of subsequent rebinds: 262.12 ms > > Full series: > > First binding: 1359 ms > > Average of subsequent rebinds: 108.36 ms > > > > dax_pmem rebind, 100 GB devdax namespace, align=2097152 > > Base(v7.1-rc6): > > First binding: 1430 ms > > Average of subsequent rebinds: 229.12 ms > > Full series: > > First binding: 1273 ms > > Average of subsequent rebinds: 100.17 ms > > The results here are impressive, but I've been having trouble replicating them > with hmm_test on my local development machines. Both an older AMD machine and > a newer Arrow Lake based machine shows ~3% worse performance with this series > applied doing ZONE_DEVICE_PRIVATE. > > This is based on measuring the memremap_pages() call when inserting > test_hmm.ko > in a VM using the following hack to measure 10 64GB memremaps. Is there an > easy > way for me to replicate your results in a VM? Or is there something in my > testing that I'm missing here? > > --- > > diff --git a/lib/test_hmm.c b/lib/test_hmm.c > index 213504915737..a1d5463dbc86 100644 > --- a/lib/test_hmm.c > +++ b/lib/test_hmm.c > @@ -34,7 +34,7 @@ > > #define DMIRROR_NDEVICES 4 > #define DMIRROR_RANGE_FAULT_TIMEOUT 1000 > -#define DEVMEM_CHUNK_SIZE (256 * 1024 * 1024U) > +#define DEVMEM_CHUNK_SIZE (64 * 1024 * 1024 * 1024UL) > #define DEVMEM_CHUNKS_RESERVE 16 > > /* > @@ -565,6 +565,8 @@ static int dmirror_allocate_chunk(struct dmirror_device > *mdevice, > unsigned long pfn_last; > void *ptr; > int ret = -ENOMEM; > + int i; > + u64 t0, total = 0; > > devmem = kzalloc_obj(*devmem); > if (!devmem) > @@ -613,6 +615,22 @@ static int dmirror_allocate_chunk(struct dmirror_device > *mdevice, > mdevice->devmem_capacity = new_capacity; > mdevice->devmem_chunks = new_chunks; > } > + > + for (i = 0; i < 10; i++) { > + t0 = ktime_get_ns(); > + ptr = memremap_pages(&devmem->pagemap, numa_node_id()); > + total += ktime_get_ns() - t0; > + if (IS_ERR_OR_NULL(ptr)) { > + if (ptr) > + ret = PTR_ERR(ptr); > + else > + ret = -EFAULT; > + goto err_release; > + } > + memunmap_pages(&devmem->pagemap); > + } > + pr_info("avg memremap %llu ns\n", total / i); > + > ptr = memremap_pages(&devmem->pagemap, numa_node_id()); > if (IS_ERR_OR_NULL(ptr)) { > if (ptr) > @@ -629,7 +647,7 @@ static int dmirror_allocate_chunk(struct dmirror_device > *mdevice, > > mutex_unlock(&mdevice->devmem_lock); > > - pr_info("added new %u MB chunk (total %u chunks, %u MB) PFNs [0x%lx > 0x%lx)\n", > + pr_info("added new %lu MB chunk (total %u chunks, %lu MB) PFNs [0x%lx > 0x%lx)\n", > DEVMEM_CHUNK_SIZE / (1024 * 1024), > mdevice->devmem_count, > mdevice->devmem_count * (DEVMEM_CHUNK_SIZE / (1024 * 1024)),
Thanks for the feedback and for sharing your test results. I reran the measurements on my side using two setups. I do not currently have access to physical PMEM hardware on my side, and the target use case for this work is a virtualized environment. So my measurements were taken in a guest using a 100G emulated pmem device backed by a regular file on the host filesystem. First, I followed your modified test_hmm.c approach, i.e. looping over memremap_pages() / memunmap_pages() and measuring the average memremap time in the MEMORY_DEVICE_PRIVATE case, where the vmemmap backing comes from normal system RAM. On this setup, I got: - base kernel: avg memremap 222.0 ms - patches 1-5 only: avg memremap 206.9 ms - full 8-patch series: avg memremap 264.1 ms I also enabled the pr_debug() timing inside memmap_init_zone_device() for the same setup, and the numbers tracked that closely: - base kernel: 221.0 ms - patches 1-5 only: 206.0 ms - full 8-patch series: 260.1 ms So on this path, patches 1-5 seem to help, but the full 8-patch series does not. Second, I also tested a benchmark-only setup corresponding to the FS_DAX map=dev case, where the memmap itself is allocated from the dax altmap range rather than normal DRAM. On that setup, I got: - base kernel: avg memremap 200.8 ms, pr_debug 196.4 ms - full 8-patch series: avg memremap 117.2 ms, pr_debug 113.5 ms So on my side, the full series shows a clear gain in the FS_DAX + altmap case, but not in the MEMORY_DEVICE_PRIVATE / DRAM-backed vmemmap case. If convenient, could you also try the same kind of measurement from my cover letter, or at least enable the pr_debug() in memmap_init_zone_device(), to check whether the delta is visible there on your setup as well? Also, if you have time, could you please try your modified test_hmm.c setup with patches 1-5 only? On my side that configuration still shows a measurable improvement. Given these results, I would also appreciate your advice on how best to evolve the series. My current understanding is that patches 1-5 are a more generic optimization, while patches 6-8 are only beneficial in some cases. Do you think patches 1-5 alone would already be a reasonable candidate for upstreaming? For patches 6-8, I am not yet sure what the right direction is. Would it make more sense to expose some explicit opt-in mechanism so that the movnti-based path is selected only when desired, or does it make more sense to use that path unconditionally for FS_DAX map=dev case? I would also be interested in your view on why the FS_DAX + altmap case shows a large gain while the DRAM-backed vmemmap case shows a regression with the full series. I do not think I fully understand that difference yet. Thanks, Zhe
