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SubjectRe: [PATCH v10 18/25] mm: provide speculative fault infrastructure
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On 15/05/2018 15:09, vinayak menon wrote:
> On Tue, Apr 17, 2018 at 8:03 PM, Laurent Dufour
> <ldufour@linux.vnet.ibm.com> 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
>>
>

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