Re: [PATCH v11 19/26] mm: provide speculative fault infrastructure

[Laurent Dufour <ldufour@linux.vnet.ibm.com>]

On 25/07/2018 11:04, zhong jiang wrote:
> On 2018/7/25 0:10, Laurent Dufour wrote:
>>
>> On 24/07/2018 16:26, zhong jiang wrote:
>>> 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.
>> Hi Zhong,
>>
>> Well this will allow file mapping to be handle in a speculative way, but that's
>> a bit dangerous today as there is no guaranty that the vm_ops.vm_fault()
>> operation will be fair.
>>
>> In the case of the anonymous file mapping that's often not a problem, depending
>> on the underlying file system, but there are so many cases to check and this is
>> hard to say this can be done in a speculative way as is.
>  This patch say that spf just handle anonyous page. but I find that do_swap_page
>  also maybe release the mmap_sem without FAULT_FLAG_RETRY_NOWAIT. why is it safe
>  to handle the case. I think that the case is similar to file page. Maybe I miss
>  something else.

do_swap_page() may released the mmap_sem through the call to
__lock_page_or_retry(), but this can only happen if FAULT_FLAG_ALLOW_RETRY or
FAULT_FLAG_KILLABLE are set and they are unset in __handle_speculative_fault().

> 
>  I test the patches and find just only 18% of the pagefault will enter into the
>  speculative page fault during a process startup. As I had said. most of pagefault
>  will be handled by ops->fault. I do not know the data you had posted is how to get.

I do agree that handling file mapping will be required, but this will add more
complexity to this series, since we need a way for drivers to tell they are
compatible with the speculative path.

May be I should give it a try on the next send.

For my information, what was the performance improvement you seen when handling
file page faulting this way ?

Thanks,
Laurent.

> 
> 
>  Thanks
>  zhong jiang
>> The huge work to do is to double check that all the code called by
>> vm_ops.fault() is not dealing with the mmap_sem, which could be handled using
>> FAULT_FLAG_RETRY_NOWAIT, and care is also needed about the resources that code
>> is managing as it may assume that it is under the protection of the mmap_sem in
>> read mode, and that can be done implicitly.
>>
>> Cheers,
>> Laurent.
>>
>>> 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
>>>>   *
>>>
>>
>> .
>>
> 
>