Messages in this thread | | | Date | Fri, 9 Jul 2010 17:12:11 +0200 | From | Andrea Arcangeli <> | Subject | Re: mmu notifier calls in apply_to_page_range() |
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On Fri, Jul 09, 2010 at 08:06:20AM -0700, Jeremy Fitzhardinge wrote: > I just noticed that the original mmu notifier change (cddb8a5c14a) adds > calls to mmu_notifier_invalidate_range_start/end to > apply_to_page_range(). This doesn't seem correct to me, since > apply_to_page_range can perform arbitrary operations to the range of > pages, not just invalidation of the pages. It seems to me that the > appropriate mmu notifiers should be called either around the call to > apply_to_page_range(), or from within the callback function. > > Andrea, what's the rationale for mmu_notifier_invalidate_range_start/end > here?
As long as the secondary mappings are teardown in range_start and allowed to be established again only after range_end, all modifications will be picked up by the secondary mmu. Imagine secondary mmu like a tlb, that you only invalidate, then it'll be refilled later (after range_end).
The exception is set_pte_at_notify that is called by ksm to establish a readonly secondary pte in KVM, KVM only calls get_user_pages(write=1) (never write=0 even for reads) so until that is optimized set_pte_at_notify allows guest to access readonly data without breaking the cow. set_pte_at_notify invokes a change_pte method, if not implemented it'll just fallback to the invalidate_page method that is backwards compatible, so no mmu notifier user is required to call change_pte (especially if the secondary page fault - kind of secondary tlb-miss software handler invokes get_user_pages with write=0 for reads, ->change_pte can only eliminate one minor fault so no big deal).
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