Messages in this thread | | | Subject | Re: [PATCH v3 0/3] A Solution to Re-enable hugetlb vmemmap optimize | From | Nanyong Sun <> | Date | Thu, 8 Feb 2024 17:44:48 +0800 |
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在 2024/2/7 20:20, Catalin Marinas 写道: > On Wed, Feb 07, 2024 at 11:21:17AM +0000, Matthew Wilcox wrote: >> On Wed, Feb 07, 2024 at 11:12:52AM +0000, Will Deacon wrote: >>> On Sat, Jan 27, 2024 at 01:04:15PM +0800, Nanyong Sun wrote: >>>> On 2024/1/26 2:06, Catalin Marinas wrote: >>>>> On Sat, Jan 13, 2024 at 05:44:33PM +0800, Nanyong Sun wrote: >>>>>> HVO was previously disabled on arm64 [1] due to the lack of necessary >>>>>> BBM(break-before-make) logic when changing page tables. >>>>>> This set of patches fix this by adding necessary BBM sequence when >>>>>> changing page table, and supporting vmemmap page fault handling to >>>>>> fixup kernel address translation fault if vmemmap is concurrently accessed. >>>>> I'm not keen on this approach. I'm not even sure it's safe. In the >>>>> second patch, you take the init_mm.page_table_lock on the fault path but >>>>> are we sure this is unlocked when the fault was taken? >>>> I think this situation is impossible. In the implementation of the second >>>> patch, when the page table is being corrupted >>>> (the time window when a page fault may occur), vmemmap_update_pte() already >>>> holds the init_mm.page_table_lock, >>>> and unlock it until page table update is done.Another thread could not hold >>>> the init_mm.page_table_lock and >>>> also trigger a page fault at the same time. >>>> If I have missed any points in my thinking, please correct me. Thank you. >>> It still strikes me as incredibly fragile to handle the fault and trying >>> to reason about all the users of 'struct page' is impossible. For example, >>> can the fault happen from irq context? >> The pte lock cannot be taken in irq context (which I think is what >> you're asking?) > With this patchset, I think it can: IRQ -> interrupt handler accesses > vmemmap -> faults -> fault handler in patch 2 takes the > init_mm.page_table_lock to wait for the vmemmap rewriting to complete. > Maybe it works if the hugetlb code disabled the IRQs but, as Will said, > such fault in any kernel context looks fragile. How about take a new lock with irq disabled during BBM, like:
+void vmemmap_update_pte(unsigned long addr, pte_t *ptep, pte_t pte) +{ + spin_lock_irq(NEW_LOCK); + pte_clear(&init_mm, addr, ptep); + flush_tlb_kernel_range(addr, addr + PAGE_SIZE); + set_pte_at(&init_mm, addr, ptep, pte); + spin_unlock_irq(NEW_LOCK); +}
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