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.
> + 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
>
