Re: [PATCH v10 18/25] mm: provide speculative fault infrastructure

[Laurent Dufour]

On 15/05/2018 15:09, vinayak menon wrote:
> On Tue, Apr 17, 2018 at 8:03 PM, Laurent Dufour
> <ldufour@xxxxxxxxxxxxxxxxxx> wrote:
>>
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> +
>> +#ifndef __HAVE_ARCH_PTE_SPECIAL
>> +/* This is required by vm_normal_page() */
>> +#error "Speculative page fault handler requires __HAVE_ARCH_PTE_SPECIAL"
>> +#endif
>> +
>> +/*
>> + * vm_normal_page() adds some processing which should be done while
>> + * hodling the mmap_sem.
>> + */
>> +int __handle_speculative_fault(struct mm_struct *mm, unsigned long address,
>> +                              unsigned int flags)
>> +{
>> +       struct vm_fault vmf = {
>> +               .address = address,
>> +       };
>> +       pgd_t *pgd, pgdval;
>> +       p4d_t *p4d, p4dval;
>> +       pud_t pudval;
>> +       int seq, ret = VM_FAULT_RETRY;
>> +       struct vm_area_struct *vma;
>> +
>> +       /* Clear flags that may lead to release the mmap_sem to retry */
>> +       flags &= ~(FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_KILLABLE);
>> +       flags |= FAULT_FLAG_SPECULATIVE;
>> +
>> +       vma = get_vma(mm, address);
>> +       if (!vma)
>> +               return ret;
>> +
>> +       seq = raw_read_seqcount(&vma->vm_sequence); /* rmb <-> seqlock,vma_rb_erase() */
>> +       if (seq & 1)
>> +               goto out_put;
>> +
>> +       /*
>> +        * Can't call vm_ops service has we don't know what they would do
>> +        * with the VMA.
>> +        * This include huge page from hugetlbfs.
>> +        */
>> +       if (vma->vm_ops)
>> +               goto out_put;
>> +
>> +       /*
>> +        * __anon_vma_prepare() requires the mmap_sem to be held
>> +        * because vm_next and vm_prev must be safe. This can't be guaranteed
>> +        * in the speculative path.
>> +        */
>> +       if (unlikely(!vma->anon_vma))
>> +               goto out_put;
>> +
>> +       vmf.vma_flags = READ_ONCE(vma->vm_flags);
>> +       vmf.vma_page_prot = READ_ONCE(vma->vm_page_prot);
>> +
>> +       /* Can't call userland page fault handler in the speculative path */
>> +       if (unlikely(vmf.vma_flags & VM_UFFD_MISSING))
>> +               goto out_put;
>> +
>> +       if (vmf.vma_flags & VM_GROWSDOWN || vmf.vma_flags & VM_GROWSUP)
>> +               /*
>> +                * This could be detected by the check address against VMA's
>> +                * boundaries but we want to trace it as not supported instead
>> +                * of changed.
>> +                */
>> +               goto out_put;
>> +
>> +       if (address < READ_ONCE(vma->vm_start)
>> +           || READ_ONCE(vma->vm_end) <= address)
>> +               goto out_put;
>> +
>> +       if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
>> +                                      flags & FAULT_FLAG_INSTRUCTION,
>> +                                      flags & FAULT_FLAG_REMOTE)) {
>> +               ret = VM_FAULT_SIGSEGV;
>> +               goto out_put;
>> +       }
>> +
>> +       /* This is one is required to check that the VMA has write access set */
>> +       if (flags & FAULT_FLAG_WRITE) {
>> +               if (unlikely(!(vmf.vma_flags & VM_WRITE))) {
>> +                       ret = VM_FAULT_SIGSEGV;
>> +                       goto out_put;
>> +               }
>> +       } else if (unlikely(!(vmf.vma_flags & (VM_READ|VM_EXEC|VM_WRITE)))) {
>> +               ret = VM_FAULT_SIGSEGV;
>> +               goto out_put;
>> +       }
>> +
>> +       if (IS_ENABLED(CONFIG_NUMA)) {
>> +               struct mempolicy *pol;
>> +
>> +               /*
>> +                * MPOL_INTERLEAVE implies additional checks in
>> +                * mpol_misplaced() which are not compatible with the
>> +                *speculative page fault processing.
>> +                */
>> +               pol = __get_vma_policy(vma, address);
> 
> 
> This gives a compile time error when CONFIG_NUMA is disabled, as there
> is no definition for
> __get_vma_policy.

IS_ENABLED is not workiing as I expected, my mistake.
I'll rollback to the legacy #ifdef stuff.

Thanks,
Laurent.


>> +               if (!pol)
>> +                       pol = get_task_policy(current);
>> +               if (pol && pol->mode == MPOL_INTERLEAVE)
>> +                       goto out_put;
>> +       }
>> +
>> +       /*
>> +        * Do a speculative lookup of the PTE entry.
>> +        */
>> +       local_irq_disable();
>> +       pgd = pgd_offset(mm, address);
>> +       pgdval = READ_ONCE(*pgd);
>> +       if (pgd_none(pgdval) || unlikely(pgd_bad(pgdval)))
>> +               goto out_walk;
>> +
>> +       p4d = p4d_offset(pgd, address);
>> +       p4dval = READ_ONCE(*p4d);
>> +       if (p4d_none(p4dval) || unlikely(p4d_bad(p4dval)))
>> +               goto out_walk;
>> +
>> +       vmf.pud = pud_offset(p4d, address);
>> +       pudval = READ_ONCE(*vmf.pud);
>> +       if (pud_none(pudval) || unlikely(pud_bad(pudval)))
>> +               goto out_walk;
>> +
>> +       /* Huge pages at PUD level are not supported. */
>> +       if (unlikely(pud_trans_huge(pudval)))
>> +               goto out_walk;
>> +
>> +       vmf.pmd = pmd_offset(vmf.pud, address);
>> +       vmf.orig_pmd = READ_ONCE(*vmf.pmd);
>> +       /*
>> +        * pmd_none could mean that a hugepage collapse is in progress
>> +        * in our back as collapse_huge_page() mark it before
>> +        * invalidating the pte (which is done once the IPI is catched
>> +        * by all CPU and we have interrupt disabled).
>> +        * For this reason we cannot handle THP in a speculative way since we
>> +        * can't safely indentify an in progress collapse operation done in our
>> +        * back on that PMD.
>> +        * Regarding the order of the following checks, see comment in
>> +        * pmd_devmap_trans_unstable()
>> +        */
>> +       if (unlikely(pmd_devmap(vmf.orig_pmd) ||
>> +                    pmd_none(vmf.orig_pmd) || pmd_trans_huge(vmf.orig_pmd) ||
>> +                    is_swap_pmd(vmf.orig_pmd)))
>> +               goto out_walk;
>> +
>> +       /*
>> +        * The above does not allocate/instantiate page-tables because doing so
>> +        * would lead to the possibility of instantiating page-tables after
>> +        * free_pgtables() -- and consequently leaking them.
>> +        *
>> +        * The result is that we take at least one !speculative fault per PMD
>> +        * in order to instantiate it.
>> +        */
>> +
>> +       vmf.pte = pte_offset_map(vmf.pmd, address);
>> +       vmf.orig_pte = READ_ONCE(*vmf.pte);
>> +       barrier(); /* See comment in handle_pte_fault() */
>> +       if (pte_none(vmf.orig_pte)) {
>> +               pte_unmap(vmf.pte);
>> +               vmf.pte = NULL;
>> +       }
>> +
>> +       vmf.vma = vma;
>> +       vmf.pgoff = linear_page_index(vma, address);
>> +       vmf.gfp_mask = __get_fault_gfp_mask(vma);
>> +       vmf.sequence = seq;
>> +       vmf.flags = flags;
>> +
>> +       local_irq_enable();
>> +
>> +       /*
>> +        * We need to re-validate the VMA after checking the bounds, otherwise
>> +        * we might have a false positive on the bounds.
>> +        */
>> +       if (read_seqcount_retry(&vma->vm_sequence, seq))
>> +               goto out_put;
>> +
>> +       mem_cgroup_oom_enable();
>> +       ret = handle_pte_fault(&vmf);
>> +       mem_cgroup_oom_disable();
>> +
>> +       put_vma(vma);
>> +
>> +       /*
>> +        * The task may have entered a memcg OOM situation but
>> +        * if the allocation error was handled gracefully (no
>> +        * VM_FAULT_OOM), there is no need to kill anything.
>> +        * Just clean up the OOM state peacefully.
>> +        */
>> +       if (task_in_memcg_oom(current) && !(ret & VM_FAULT_OOM))
>> +               mem_cgroup_oom_synchronize(false);
>> +       return ret;
>> +
>> +out_walk:
>> +       local_irq_enable();
>> +out_put:
>> +       put_vma(vma);
>> +       return ret;
>> +}
>> +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
>> +
>>  /*
>>   * By the time we get here, we already hold the mm semaphore
>>   *
>> --
>> 2.7.4
>>
>