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Date:   Tue, 24 Jul 2018 22:26:29 +0800
From:   zhong jiang <zhongjiang@huawei.com>
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To:     Laurent Dufour <ldufour@linux.vnet.ibm.com>
CC:     <akpm@linux-foundation.org>, <mhocko@kernel.org>,
        <peterz@infradead.org>, <kirill@shutemov.name>,
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        Tim Chen <tim.c.chen@linux.intel.com>,
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Subject: Re: [PATCH v11 19/26] mm: provide speculative fault infrastructure
References: <1526555193-7242-1-git-send-email-ldufour@linux.vnet.ibm.com> <1526555193-7242-20-git-send-email-ldufour@linux.vnet.ibm.com>
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On 2018/5/17 19:06, Laurent Dufour wrote:
> From: Peter Zijlstra <peterz@infradead.org>
>
> Provide infrastructure to do a speculative fault (not holding
> mmap_sem).
>
> The not holding of mmap_sem means we can race against VMA
> change/removal and page-table destruction. We use the SRCU VMA freeing
> to keep the VMA around. We use the VMA seqcount to detect change
> (including umapping / page-table deletion) and we use gup_fast() style
> page-table walking to deal with page-table races.
>
> Once we've obtained the page and are ready to update the PTE, we
> validate if the state we started the fault with is still valid, if
> not, we'll fail the fault with VM_FAULT_RETRY, otherwise we update the
> PTE and we're done.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>
> [Manage the newly introduced pte_spinlock() for speculative page
>  fault to fail if the VMA is touched in our back]
> [Rename vma_is_dead() to vma_has_changed() and declare it here]
> [Fetch p4d and pud]
> [Set vmd.sequence in __handle_mm_fault()]
> [Abort speculative path when handle_userfault() has to be called]
> [Add additional VMA's flags checks in handle_speculative_fault()]
> [Clear FAULT_FLAG_ALLOW_RETRY in handle_speculative_fault()]
> [Don't set vmf->pte and vmf->ptl if pte_map_lock() failed]
> [Remove warning comment about waiting for !seq&1 since we don't want
>  to wait]
> [Remove warning about no huge page support, mention it explictly]
> [Don't call do_fault() in the speculative path as __do_fault() calls
>  vma->vm_ops->fault() which may want to release mmap_sem]
> [Only vm_fault pointer argument for vma_has_changed()]
> [Fix check against huge page, calling pmd_trans_huge()]
> [Use READ_ONCE() when reading VMA's fields in the speculative path]
> [Explicitly check for __HAVE_ARCH_PTE_SPECIAL as we can't support for
>  processing done in vm_normal_page()]
> [Check that vma->anon_vma is already set when starting the speculative
>  path]
> [Check for memory policy as we can't support MPOL_INTERLEAVE case due to
>  the processing done in mpol_misplaced()]
> [Don't support VMA growing up or down]
> [Move check on vm_sequence just before calling handle_pte_fault()]
> [Don't build SPF services if !CONFIG_SPECULATIVE_PAGE_FAULT]
> [Add mem cgroup oom check]
> [Use READ_ONCE to access p*d entries]
> [Replace deprecated ACCESS_ONCE() by READ_ONCE() in vma_has_changed()]
> [Don't fetch pte again in handle_pte_fault() when running the speculative
>  path]
> [Check PMD against concurrent collapsing operation]
> [Try spin lock the pte during the speculative path to avoid deadlock with
>  other CPU's invalidating the TLB and requiring this CPU to catch the
>  inter processor's interrupt]
> [Move define of FAULT_FLAG_SPECULATIVE here]
> [Introduce __handle_speculative_fault() and add a check against
>  mm->mm_users in handle_speculative_fault() defined in mm.h]
> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
> ---
>  include/linux/hugetlb_inline.h |   2 +-
>  include/linux/mm.h             |  30 ++++
>  include/linux/pagemap.h        |   4 +-
>  mm/internal.h                  |  16 +-
>  mm/memory.c                    | 340 ++++++++++++++++++++++++++++++++++++++++-
>  5 files changed, 385 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/hugetlb_inline.h b/include/linux/hugetlb_inline.h
> index 0660a03d37d9..9e25283d6fc9 100644
> --- a/include/linux/hugetlb_inline.h
> +++ b/include/linux/hugetlb_inline.h
> @@ -8,7 +8,7 @@
>  
>  static inline bool is_vm_hugetlb_page(struct vm_area_struct *vma)
>  {
> -	return !!(vma->vm_flags & VM_HUGETLB);
> +	return !!(READ_ONCE(vma->vm_flags) & VM_HUGETLB);
>  }
>  
>  #else
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 05cbba70104b..31acf98a7d92 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -315,6 +315,7 @@ extern pgprot_t protection_map[16];
>  #define FAULT_FLAG_USER		0x40	/* The fault originated in userspace */
>  #define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
>  #define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
> +#define FAULT_FLAG_SPECULATIVE	0x200	/* Speculative fault, not holding mmap_sem */
>  
>  #define FAULT_FLAG_TRACE \
>  	{ FAULT_FLAG_WRITE,		"WRITE" }, \
> @@ -343,6 +344,10 @@ struct vm_fault {
>  	gfp_t gfp_mask;			/* gfp mask to be used for allocations */
>  	pgoff_t pgoff;			/* Logical page offset based on vma */
>  	unsigned long address;		/* Faulting virtual address */
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +	unsigned int sequence;
> +	pmd_t orig_pmd;			/* value of PMD at the time of fault */
> +#endif
>  	pmd_t *pmd;			/* Pointer to pmd entry matching
>  					 * the 'address' */
>  	pud_t *pud;			/* Pointer to pud entry matching
> @@ -1415,6 +1420,31 @@ int invalidate_inode_page(struct page *page);
>  #ifdef CONFIG_MMU
>  extern int handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  		unsigned int flags);
> +
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +extern int __handle_speculative_fault(struct mm_struct *mm,
> +				      unsigned long address,
> +				      unsigned int flags);
> +static inline int handle_speculative_fault(struct mm_struct *mm,
> +					   unsigned long address,
> +					   unsigned int flags)
> +{
> +	/*
> +	 * Try speculative page fault for multithreaded user space task only.
> +	 */
> +	if (!(flags & FAULT_FLAG_USER) || atomic_read(&mm->mm_users) == 1)
> +		return VM_FAULT_RETRY;
> +	return __handle_speculative_fault(mm, address, flags);
> +}
> +#else
> +static inline int handle_speculative_fault(struct mm_struct *mm,
> +					   unsigned long address,
> +					   unsigned int flags)
> +{
> +	return VM_FAULT_RETRY;
> +}
> +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
> +
>  extern int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
>  			    unsigned long address, unsigned int fault_flags,
>  			    bool *unlocked);
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index b1bd2186e6d2..6e2aa4e79af7 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -456,8 +456,8 @@ static inline pgoff_t linear_page_index(struct vm_area_struct *vma,
>  	pgoff_t pgoff;
>  	if (unlikely(is_vm_hugetlb_page(vma)))
>  		return linear_hugepage_index(vma, address);
> -	pgoff = (address - vma->vm_start) >> PAGE_SHIFT;
> -	pgoff += vma->vm_pgoff;
> +	pgoff = (address - READ_ONCE(vma->vm_start)) >> PAGE_SHIFT;
> +	pgoff += READ_ONCE(vma->vm_pgoff);
>  	return pgoff;
>  }
>  
> diff --git a/mm/internal.h b/mm/internal.h
> index fb2667b20f0a..10b188c87fa4 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -44,7 +44,21 @@ int do_swap_page(struct vm_fault *vmf);
>  extern struct vm_area_struct *get_vma(struct mm_struct *mm,
>  				      unsigned long addr);
>  extern void put_vma(struct vm_area_struct *vma);
> -#endif
> +
> +static inline bool vma_has_changed(struct vm_fault *vmf)
> +{
> +	int ret = RB_EMPTY_NODE(&vmf->vma->vm_rb);
> +	unsigned int seq = READ_ONCE(vmf->vma->vm_sequence.sequence);
> +
> +	/*
> +	 * Matches both the wmb in write_seqlock_{begin,end}() and
> +	 * the wmb in vma_rb_erase().
> +	 */
> +	smp_rmb();
> +
> +	return ret || seq != vmf->sequence;
> +}
> +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
>  
>  void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>  		unsigned long floor, unsigned long ceiling);
> diff --git a/mm/memory.c b/mm/memory.c
> index ab32b0b4bd69..7bbbb8c7b9cd 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -769,7 +769,8 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
>  	if (page)
>  		dump_page(page, "bad pte");
>  	pr_alert("addr:%p vm_flags:%08lx anon_vma:%p mapping:%p index:%lx\n",
> -		 (void *)addr, vma->vm_flags, vma->anon_vma, mapping, index);
> +		 (void *)addr, READ_ONCE(vma->vm_flags), vma->anon_vma,
> +		 mapping, index);
>  	pr_alert("file:%pD fault:%pf mmap:%pf readpage:%pf\n",
>  		 vma->vm_file,
>  		 vma->vm_ops ? vma->vm_ops->fault : NULL,
> @@ -2306,6 +2307,118 @@ int apply_to_page_range(struct mm_struct *mm, unsigned long addr,
>  }
>  EXPORT_SYMBOL_GPL(apply_to_page_range);
>  
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +static bool pte_spinlock(struct vm_fault *vmf)
> +{
> +	bool ret = false;
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	pmd_t pmdval;
> +#endif
> +
> +	/* Check if vma is still valid */
> +	if (!(vmf->flags & FAULT_FLAG_SPECULATIVE)) {
> +		vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> +		spin_lock(vmf->ptl);
> +		return true;
> +	}
> +
> +again:
> +	local_irq_disable();
> +	if (vma_has_changed(vmf))
> +		goto out;
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	/*
> +	 * We check if the pmd value is still the same to ensure that there
> +	 * is not a huge collapse operation in progress in our back.
> +	 */
> +	pmdval = READ_ONCE(*vmf->pmd);
> +	if (!pmd_same(pmdval, vmf->orig_pmd))
> +		goto out;
> +#endif
> +
> +	vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> +	if (unlikely(!spin_trylock(vmf->ptl))) {
> +		local_irq_enable();
> +		goto again;
> +	}
> +
> +	if (vma_has_changed(vmf)) {
> +		spin_unlock(vmf->ptl);
> +		goto out;
> +	}
> +
> +	ret = true;
> +out:
> +	local_irq_enable();
> +	return ret;
> +}
> +
> +static bool pte_map_lock(struct vm_fault *vmf)
> +{
> +	bool ret = false;
> +	pte_t *pte;
> +	spinlock_t *ptl;
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	pmd_t pmdval;
> +#endif
> +
> +	if (!(vmf->flags & FAULT_FLAG_SPECULATIVE)) {
> +		vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
> +					       vmf->address, &vmf->ptl);
> +		return true;
> +	}
> +
> +	/*
> +	 * The first vma_has_changed() guarantees the page-tables are still
> +	 * valid, having IRQs disabled ensures they stay around, hence the
> +	 * second vma_has_changed() to make sure they are still valid once
> +	 * we've got the lock. After that a concurrent zap_pte_range() will
> +	 * block on the PTL and thus we're safe.
> +	 */
> +again:
> +	local_irq_disable();
> +	if (vma_has_changed(vmf))
> +		goto out;
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	/*
> +	 * We check if the pmd value is still the same to ensure that there
> +	 * is not a huge collapse operation in progress in our back.
> +	 */
> +	pmdval = READ_ONCE(*vmf->pmd);
> +	if (!pmd_same(pmdval, vmf->orig_pmd))
> +		goto out;
> +#endif
> +
> +	/*
> +	 * Same as pte_offset_map_lock() except that we call
> +	 * spin_trylock() in place of spin_lock() to avoid race with
> +	 * unmap path which may have the lock and wait for this CPU
> +	 * to invalidate TLB but this CPU has irq disabled.
> +	 * Since we are in a speculative patch, accept it could fail
> +	 */
> +	ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> +	pte = pte_offset_map(vmf->pmd, vmf->address);
> +	if (unlikely(!spin_trylock(ptl))) {
> +		pte_unmap(pte);
> +		local_irq_enable();
> +		goto again;
> +	}
> +
> +	if (vma_has_changed(vmf)) {
> +		pte_unmap_unlock(pte, ptl);
> +		goto out;
> +	}
> +
> +	vmf->pte = pte;
> +	vmf->ptl = ptl;
> +	ret = true;
> +out:
> +	local_irq_enable();
> +	return ret;
> +}
> +#else
>  static inline bool pte_spinlock(struct vm_fault *vmf)
>  {
>  	vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> @@ -2319,6 +2432,7 @@ static inline bool pte_map_lock(struct vm_fault *vmf)
>  				       vmf->address, &vmf->ptl);
>  	return true;
>  }
> +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
>  
>  /*
>   * handle_pte_fault chooses page fault handler according to an entry which was
> @@ -3208,6 +3322,14 @@ static int do_anonymous_page(struct vm_fault *vmf)
>  		ret = check_stable_address_space(vma->vm_mm);
>  		if (ret)
>  			goto unlock;
> +		/*
> +		 * Don't call the userfaultfd during the speculative path.
> +		 * We already checked for the VMA to not be managed through
> +		 * userfaultfd, but it may be set in our back once we have lock
> +		 * the pte. In such a case we can ignore it this time.
> +		 */
> +		if (vmf->flags & FAULT_FLAG_SPECULATIVE)
> +			goto setpte;
>  		/* Deliver the page fault to userland, check inside PT lock */
>  		if (userfaultfd_missing(vma)) {
>  			pte_unmap_unlock(vmf->pte, vmf->ptl);
> @@ -3249,7 +3371,7 @@ static int do_anonymous_page(struct vm_fault *vmf)
>  		goto unlock_and_release;
>  
>  	/* Deliver the page fault to userland, check inside PT lock */
> -	if (userfaultfd_missing(vma)) {
> +	if (!(vmf->flags & FAULT_FLAG_SPECULATIVE) && userfaultfd_missing(vma)) {
>  		pte_unmap_unlock(vmf->pte, vmf->ptl);
>  		mem_cgroup_cancel_charge(page, memcg, false);
>  		put_page(page);
> @@ -3994,13 +4116,22 @@ static int handle_pte_fault(struct vm_fault *vmf)
>  
>  	if (unlikely(pmd_none(*vmf->pmd))) {
>  		/*
> +		 * In the case of the speculative page fault handler we abort
> +		 * the speculative path immediately as the pmd is probably
> +		 * in the way to be converted in a huge one. We will try
> +		 * again holding the mmap_sem (which implies that the collapse
> +		 * operation is done).
> +		 */
> +		if (vmf->flags & FAULT_FLAG_SPECULATIVE)
> +			return VM_FAULT_RETRY;
> +		/*
>  		 * Leave __pte_alloc() until later: because vm_ops->fault may
>  		 * want to allocate huge page, and if we expose page table
>  		 * for an instant, it will be difficult to retract from
>  		 * concurrent faults and from rmap lookups.
>  		 */
>  		vmf->pte = NULL;
> -	} else {
> +	} else if (!(vmf->flags & FAULT_FLAG_SPECULATIVE)) {
>  		/* See comment in pte_alloc_one_map() */
>  		if (pmd_devmap_trans_unstable(vmf->pmd))
>  			return 0;
> @@ -4009,6 +4140,9 @@ static int handle_pte_fault(struct vm_fault *vmf)
>  		 * pmd from under us anymore at this point because we hold the
>  		 * mmap_sem read mode and khugepaged takes it in write mode.
>  		 * So now it's safe to run pte_offset_map().
> +		 * This is not applicable to the speculative page fault handler
> +		 * but in that case, the pte is fetched earlier in
> +		 * handle_speculative_fault().
>  		 */
>  		vmf->pte = pte_offset_map(vmf->pmd, vmf->address);
>  		vmf->orig_pte = *vmf->pte;
> @@ -4031,6 +4165,8 @@ static int handle_pte_fault(struct vm_fault *vmf)
>  	if (!vmf->pte) {
>  		if (vma_is_anonymous(vmf->vma))
>  			return do_anonymous_page(vmf);
> +		else if (vmf->flags & FAULT_FLAG_SPECULATIVE)
> +			return VM_FAULT_RETRY;
>  		else
>  			return do_fault(vmf);
>  	}
> @@ -4128,6 +4264,9 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  	vmf.pmd = pmd_alloc(mm, vmf.pud, address);
>  	if (!vmf.pmd)
>  		return VM_FAULT_OOM;
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +	vmf.sequence = raw_read_seqcount(&vma->vm_sequence);
> +#endif
>  	if (pmd_none(*vmf.pmd) && transparent_hugepage_enabled(vma)) {
>  		ret = create_huge_pmd(&vmf);
>  		if (!(ret & VM_FAULT_FALLBACK))
> @@ -4161,6 +4300,201 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  	return handle_pte_fault(&vmf);
>  }
>  
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +/*
> + * Tries to handle the page fault in a speculative way, without grabbing 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;
> +#ifdef CONFIG_NUMA
> +	struct mempolicy *pol;
> +#endif
> +
> +	/* 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;
> +
  Hi   Laurent
   
   I think that most of pagefault will leave here.   Is there any case  need to skip ?
  I have tested the following  patch, it work well.

diff --git a/mm/memory.c b/mm/memory.c
index 936128b..9bc1545 100644
 @@ -3893,8 +3898,6 @@ static int handle_pte_fault(struct fault_env *fe)
        if (!fe->pte) {
                if (vma_is_anonymous(fe->vma))
                        return do_anonymous_page(fe);
-               else if (fe->flags & FAULT_FLAG_SPECULATIVE)
-                       return VM_FAULT_RETRY;
                else
                        return do_fault(fe);
        }
@@ -4026,20 +4029,11 @@ int __handle_speculative_fault(struct mm_struct *mm, unsigned long address,
                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) {
-               trace_spf_vma_notsup(_RET_IP_, vma, address);
-               goto out_put;
-       }


Thanks
zhong jiang
> +	/*
> +	 * __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;
> +	}
> +
> +#ifdef CONFIG_NUMA
> +	/*
> +	 * MPOL_INTERLEAVE implies additional checks in
> +	 * mpol_misplaced() which are not compatible with the
> +	 *speculative page fault processing.
> +	 */
> +	pol = __get_vma_policy(vma, address);
> +	if (!pol)
> +		pol = get_task_policy(current);
> +	if (pol && pol->mode == MPOL_INTERLEAVE)
> +		goto out_put;
> +#endif
> +
> +	/*
> +	 * 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
>   *