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vm_area_struct *vma, >> unsigned long address, pte_t *ptep) >> { >> - int young = ptep_test_and_clear_young(vma, address, ptep); >> - >> - if (young) { >> - /* >> - * We can elide the trailing DSB here since the worst that can >> - * happen is that a CPU continues to use the young entry in its >> - * TLB and we mistakenly reclaim the associated page. The >> - * window for such an event is bounded by the next >> - * context-switch, which provides a DSB to complete the TLB >> - * invalidation. >> - */ >> - flush_tlb_page_nosync(vma, address); >> - } >> - >> - return young; >> + /* >> + * This comment is borrowed from x86, but applies equally to ARM64: >> + * >> + * Clearing the accessed bit without a TLB flush doesn't cause >> + * data corruption. [ It could cause incorrect page aging and >> + * the (mistaken) reclaim of hot pages, but the chance of that >> + * should be relatively low. ] >> + * >> + * So as a performance optimization don't flush the TLB when >> + * clearing the accessed bit, it will eventually be flushed by >> + * a context switch or a VM operation anyway. [ In the rare >> + * event of it not getting flushed for a long time the delay >> + * shouldn't really matter because there's no real memory >> + * pressure for swapout to react to. ] >> + */ >> + return ptep_test_and_clear_young(vma, address, ptep); >> } >From https://lore.kernel.org/lkml/20181029105515.GD14127@arm.com/: This is blindly copied from x86 and isn't true for us: we don't invalidate the TLB on context switch. That means our window for keeping the stale entries around is potentially much bigger and might not be a great idea. My understanding is that arm64 doesn't do invalidate the TLB during context switch. The flush_tlb_page_nosync() here + DSB during context switch make sure the TLB is invalidated during context switch. So we can't remove flush_tlb_page_nosync() here? Or something was changed for arm64 (I have zero knowledge to TLB on arm64. So some obvious thing may be missed)? Thanks. Regards Yin, Fengwei