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Use proper caution when opening attachments, clicking links, or responding. > > > On 4/22/26 12:26, Hrushikesh Salunke wrote: >> When init_on_alloc is enabled, kernel_init_pages() clears every page >> one at a time via clear_highpage_kasan_tagged(), which incurs per-page >> kmap_local_page()/kunmap_local() overhead and prevents the architecture >> clearing primitive from operating on contiguous ranges. >> >> Introduce clear_highpages_kasan_tagged() in highmem.h, a batch >> clearing helper that calls clear_pages() for the full contiguous range >> on !HIGHMEM systems, bypassing the per-page kmap overhead and allowing >> a single invocation of the arch clearing primitive across the entire >> allocation. The HIGHMEM path falls back to per-page clearing since >> those pages require kmap. >> >> Replace kernel_init_pages() with direct calls to the new helper, as it >> becomes a trivial wrapper. >> >> Allocating 8192 x 2MB HugeTLB pages (16GB) with init_on_alloc=1: >> >> Before: 0.445s >> After: 0.166s (-62.7%, 2.68x faster) >> >> Kernel time (sys) reduction per workload with init_on_alloc=1: >> >> Workload Before After Change >> Graph500 64C128T 30m 41.8s 15m 14.8s -50.3% >> Graph500 16C32T 15m 56.7s 9m 43.7s -39.0% >> Pagerank 32T 1m 58.5s 1m 12.8s -38.5% >> Pagerank 128T 2m 36.3s 1m 40.4s -35.7% > We do have some elaborate handling in clear_contig_highpages() to chunk it up > (and to call cond_resched()). But that function can get called with much bigger > ranges. > > I'm not concerned about the cond_resched() -- we wouldn't do one here before -- > but I'm wondering whether we could end up triggering a HW instruction that is > uninterruptible and takes a rather long time. > > But clear_contig_highpages() breaks it into 32MiB chunks, and only x86 supports > it so far. So we won't exceed that with the maximum buddy order of 4MiB on x86. > > Acked-by: David Hildenbrand (Arm) > > -- > Cheers, > > David Right, on x86 the max buddy order keeps it well within safe limits. Also, rep stosb/stosq on x86, currently used for clearing, is interruptible, the CPU can take interrupts between iterations and resume where it left off. So even for larger ranges it wouldn't be a single uninterruptible operation. Other architectures use a per-page loop for clearing, so the same applies there. Thanks for the Ack! Regards, Hrushikesh