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

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
>> *
>
>

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