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SubjectRe: [PATCH v10 06/25] mm: make pte_unmap_same compatible with SPF
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On 23/04/2018 08:31, Minchan Kim wrote:
> On Tue, Apr 17, 2018 at 04:33:12PM +0200, Laurent Dufour 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().
>
> I don't see such logic in this patch.
> Maybe you introduces it later? If so, please comment on it.
> Or just return 0 in case of 2 without introducing VM_FAULT_PTNOTSAME.

Late in the series, pte_spinlock() will check for the VMA's changes and may
return 1. This will be then required to handle the 3 cases presented above.

I can move this handling later in the series, but I wondering if this will make
it more easier to read.

>
>> 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@google.com>
>> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
>> ---
>> 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 */
>>
>> #define VM_FAULT_ERROR (VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | \
>> VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 0b9a51f80e0e..f86efcb8e268 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2309,21 +2309,29 @@ static inline bool pte_map_lock(struct vm_fault *vmf)
>> * parts, do_swap_page must check under lock before unmapping the pte and
>> * proceeding (but do_wp_page is only called after already making such a check;
>> * and do_anonymous_page can safely check later on).
>> + *
>> + * pte_unmap_same() returns:
>> + * 0 if the PTE are the same
>> + * VM_FAULT_PTNOTSAME if the PTE are different
>> + * VM_FAULT_RETRY if the VMA has changed in our back during
>> + * a speculative page fault handling.
>> */
>> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
>> - pte_t *page_table, pte_t orig_pte)
>> +static inline int pte_unmap_same(struct vm_fault *vmf)
>> {
>> - int same = 1;
>> + int ret = 0;
>> +
>> #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>> if (sizeof(pte_t) > sizeof(unsigned long)) {
>> - spinlock_t *ptl = pte_lockptr(mm, pmd);
>> - spin_lock(ptl);
>> - same = pte_same(*page_table, orig_pte);
>> - spin_unlock(ptl);
>> + if (pte_spinlock(vmf)) {
>> + if (!pte_same(*vmf->pte, vmf->orig_pte))
>> + ret = VM_FAULT_PTNOTSAME;
>> + spin_unlock(vmf->ptl);
>> + } else
>> + ret = VM_FAULT_RETRY;
>> }
>> #endif
>> - pte_unmap(page_table);
>> - return same;
>> + pte_unmap(vmf->pte);
>> + return ret;
>> }
>>
>> static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
>> @@ -2912,10 +2920,19 @@ int do_swap_page(struct vm_fault *vmf)
>> pte_t pte;
>> int locked;
>> int exclusive = 0;
>> - int ret = 0;
>> + int ret;
>>
>> - if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
>> + ret = pte_unmap_same(vmf);
>> + if (ret) {
>> + /*
>> + * If pte != orig_pte, this means another thread did the
>> + * swap operation in our back.
>> + * So nothing else to do.
>> + */
>> + if (ret == VM_FAULT_PTNOTSAME)
>> + ret = 0;
>> goto out;
>> + }
>>
>> entry = pte_to_swp_entry(vmf->orig_pte);
>> if (unlikely(non_swap_entry(entry))) {
>> --
>> 2.7.4
>>
>

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