On 04/18/2013 07:38 PM, Gleb Natapov wrote: > On Thu, Apr 18, 2013 at 07:22:23PM +0800, Xiao Guangrong wrote: >> On 04/18/2013 07:00 PM, Gleb Natapov wrote: >>> On Tue, Apr 16, 2013 at 02:32:46PM +0800, Xiao Guangrong wrote: >>>> pte_list_clear_concurrently allows us to reset pte-desc entry >>>> out of mmu-lock. We can reset spte out of mmu-lock if we can protect the >>>> lifecycle of sp, we use this way to achieve the goal: >>>> >>>> unmap_memslot_rmap_nolock(): >>>> for-each-rmap-in-slot: >>>> preempt_disable >>>> kvm->arch.being_unmapped_rmap = rmapp >>>> clear spte and reset rmap entry >>>> kvm->arch.being_unmapped_rmap = NULL >>>> preempt_enable >>>> >>>> Other patch like zap-sp and mmu-notify which are protected >>>> by mmu-lock: >>>> clear spte and reset rmap entry >>>> retry: >>>> if (kvm->arch.being_unmapped_rmap == rmap) >>>> goto retry >>>> (the wait is very rare and clear one rmap is very fast, it >>>> is not bad even if wait is needed) >>>> >>> I do not understand what how this achieve the goal. Suppose that rmap >>> == X and kvm->arch.being_unmapped_rmap == NULL so "goto retry" is skipped, >>> but moment later unmap_memslot_rmap_nolock() does >>> vm->arch.being_unmapped_rmap = X. >> >> Access rmap is always safe since rmap and its entries are valid until >> memslot is destroyed. >> >> This algorithm protects spte since it can be freed in the protection of >> mmu-lock. >> >> In your scenario: >> >> ====== >> CPU 1 CPU 2 >> >> vcpu / mmu-notify access the RMAP unmap rmap out of mmu-lock which >> is under >> which is under mmu-lock slot-lock >> >> zap spte1 >> clear RMAP entry >> >> kvm->arch.being_unmapped_rmap = NULL, >> do not wait >> >> free spte1 >> >> set kvm->arch.being_unmapped_rmap = >> RMAP >> walking RMAP and do not see spet1 on >> RMAP >> (the entry of spte 1 has been reset >> by CPU 1) > and what prevents this from happening concurrently with "clear RMAP > entry"? Is it safe?
All the possible changes on the RMAP entry is from valid-spte to PTE_LIST_SPTE_SKIP. (no valid-spte to valid-spte / no spte to new-spte) There are three possible cases: case 1): both two paths can see the valid-spte. the worst case is, the host page can be double A/D tracked (multi calling of kvm_set_pfn_accessed/kvm_set_pfn_dirty), it is safe. case 2): only the path under protection of mmu-lock see the valid-spte this is safe since RMAP and spte are always valid under mmu-lock case 3): only the path out of mmu-lock see the valid-spte then the path under mmu-lock will being wait until the no-lock path has finished. The spte is valid and no-lock path is safe to call kvm_set_pfn_accessed/kvm_set_pfn_dirty. Do you get any potential issue? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html