On 10/05/2018 18:15, vinayak menon wrote: > On Tue, Apr 17, 2018 at 8:03 PM, Laurent Dufour > <ldufour@xxxxxxxxxxxxxxxxxx> wrote: >> pte_unmap_same() is making the assumption that the page table are still >> around because the mmap_sem is held. >> This is no more the case when running a speculative page fault and >> additional check must be made to ensure that the final page table are still >> there. >> >> This is now done by calling pte_spinlock() to check for the VMA's >> consistency while locking for the page tables. >> >> This is requiring passing a vm_fault structure to pte_unmap_same() which is >> containing all the needed parameters. >> >> As pte_spinlock() may fail in the case of a speculative page fault, if the >> VMA has been touched in our back, pte_unmap_same() should now return 3 >> cases : >> 1. pte are the same (0) >> 2. pte are different (VM_FAULT_PTNOTSAME) >> 3. a VMA's changes has been detected (VM_FAULT_RETRY) >> >> The case 2 is handled by the introduction of a new VM_FAULT flag named >> VM_FAULT_PTNOTSAME which is then trapped in cow_user_page(). >> If VM_FAULT_RETRY is returned, it is passed up to the callers to retry the >> page fault while holding the mmap_sem. >> >> Acked-by: David Rientjes <rientjes@xxxxxxxxxx> >> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx> >> --- >> include/linux/mm.h | 1 + >> mm/memory.c | 39 ++++++++++++++++++++++++++++----------- >> 2 files changed, 29 insertions(+), 11 deletions(-) >> >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index 4d1aff80669c..714da99d77a3 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -1208,6 +1208,7 @@ static inline void clear_page_pfmemalloc(struct page *page) >> #define VM_FAULT_NEEDDSYNC 0x2000 /* ->fault did not modify page tables >> * and needs fsync() to complete (for >> * synchronous page faults in DAX) */ >> +#define VM_FAULT_PTNOTSAME 0x4000 /* Page table entries have changed */ > > > This has to be added to VM_FAULT_RESULT_TRACE ? Indeed there is no chance that the macro VM_FAULT_RESULT_TRACE would have to translate that code to a string since VM_FAULT_PTNOTSAME is currently only returned by pte_unmap_same() and then converted by its only caller do_swap_page() to return 0. So VM_FAULT_PTNOTSAME is not expected to be seen outside of these services which are never using VM_FAULT_RESULT_TRACE(). This being said, this may be a good idea to add it in the case of future potential usage.
