| When do you need to notify inside page table lock ? |
| |
| When clearing a pte/pmd we are given a choice to notify the event through |
| (notify version of *_clear_flush call mmu_notifier_invalidate_range) under |
| the page table lock. But that notification is not necessary in all cases. |
| |
| For secondary TLB (non CPU TLB) like IOMMU TLB or device TLB (when device use |
| thing like ATS/PASID to get the IOMMU to walk the CPU page table to access a |
| process virtual address space). There is only 2 cases when you need to notify |
| those secondary TLB while holding page table lock when clearing a pte/pmd: |
| |
| A) page backing address is free before mmu_notifier_invalidate_range_end() |
| B) a page table entry is updated to point to a new page (COW, write fault |
| on zero page, __replace_page(), ...) |
| |
| Case A is obvious you do not want to take the risk for the device to write to |
| a page that might now be used by some completely different task. |
| |
| Case B is more subtle. For correctness it requires the following sequence to |
| happen: |
| - take page table lock |
| - clear page table entry and notify ([pmd/pte]p_huge_clear_flush_notify()) |
| - set page table entry to point to new page |
| |
| If clearing the page table entry is not followed by a notify before setting |
| the new pte/pmd value then you can break memory model like C11 or C++11 for |
| the device. |
| |
| Consider the following scenario (device use a feature similar to ATS/PASID): |
| |
| Two address addrA and addrB such that |addrA - addrB| >= PAGE_SIZE we assume |
| they are write protected for COW (other case of B apply too). |
| |
| [Time N] -------------------------------------------------------------------- |
| CPU-thread-0 {try to write to addrA} |
| CPU-thread-1 {try to write to addrB} |
| CPU-thread-2 {} |
| CPU-thread-3 {} |
| DEV-thread-0 {read addrA and populate device TLB} |
| DEV-thread-2 {read addrB and populate device TLB} |
| [Time N+1] ------------------------------------------------------------------ |
| CPU-thread-0 {COW_step0: {mmu_notifier_invalidate_range_start(addrA)}} |
| CPU-thread-1 {COW_step0: {mmu_notifier_invalidate_range_start(addrB)}} |
| CPU-thread-2 {} |
| CPU-thread-3 {} |
| DEV-thread-0 {} |
| DEV-thread-2 {} |
| [Time N+2] ------------------------------------------------------------------ |
| CPU-thread-0 {COW_step1: {update page table to point to new page for addrA}} |
| CPU-thread-1 {COW_step1: {update page table to point to new page for addrB}} |
| CPU-thread-2 {} |
| CPU-thread-3 {} |
| DEV-thread-0 {} |
| DEV-thread-2 {} |
| [Time N+3] ------------------------------------------------------------------ |
| CPU-thread-0 {preempted} |
| CPU-thread-1 {preempted} |
| CPU-thread-2 {write to addrA which is a write to new page} |
| CPU-thread-3 {} |
| DEV-thread-0 {} |
| DEV-thread-2 {} |
| [Time N+3] ------------------------------------------------------------------ |
| CPU-thread-0 {preempted} |
| CPU-thread-1 {preempted} |
| CPU-thread-2 {} |
| CPU-thread-3 {write to addrB which is a write to new page} |
| DEV-thread-0 {} |
| DEV-thread-2 {} |
| [Time N+4] ------------------------------------------------------------------ |
| CPU-thread-0 {preempted} |
| CPU-thread-1 {COW_step3: {mmu_notifier_invalidate_range_end(addrB)}} |
| CPU-thread-2 {} |
| CPU-thread-3 {} |
| DEV-thread-0 {} |
| DEV-thread-2 {} |
| [Time N+5] ------------------------------------------------------------------ |
| CPU-thread-0 {preempted} |
| CPU-thread-1 {} |
| CPU-thread-2 {} |
| CPU-thread-3 {} |
| DEV-thread-0 {read addrA from old page} |
| DEV-thread-2 {read addrB from new page} |
| |
| So here because at time N+2 the clear page table entry was not pair with a |
| notification to invalidate the secondary TLB, the device see the new value for |
| addrB before seing the new value for addrA. This break total memory ordering |
| for the device. |
| |
| When changing a pte to write protect or to point to a new write protected page |
| with same content (KSM) it is fine to delay the mmu_notifier_invalidate_range |
| call to mmu_notifier_invalidate_range_end() outside the page table lock. This |
| is true even if the thread doing the page table update is preempted right after |
| releasing page table lock but before call mmu_notifier_invalidate_range_end(). |
