[PATCH v11 19/26] mm: provide speculative fault infrastructure
zhong jiang
zhongjiang at huawei.com
Wed Jul 25 21:23:10 AEST 2018
On 2018/7/25 18:44, Laurent Dufour wrote:
>
> 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 at 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 at 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 at 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().
For spf. Indeed. Thank you for clarification.
>> 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.
As the above mentioned. the specualtive page fault do not pass FAULT_FLAG_ALLOW_RETRY.
In other words, File page will not refer to release mmap_sem for spf.
but I am still not quite clear that what should drivers do to compatible with speculatve path.
The speculative path should not refer to the mmap_sem for filemap_fault.
Thanks,
zhong jiang
> May be I should give it a try on the next send.
Ok, I will try.
> For my information, what was the performance improvement you seen when handling
> file page faulting this way ?
I am sorry that. It is the data that Ganesh test the launch time on Andriod.
> 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
>>>>> *
>>> .
>>>
>>
>
> .
>
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