Re: [PATCH v2 3/9] mm: Move FAULT_FLAG_VMA_LOCK check from handle_mm_fault()

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On Tue, Jul 11, 2023 at 1:20 PM Matthew Wilcox (Oracle)
<willy@xxxxxxxxxxxxx> wrote:
>
> Handle a little more of the page fault path outside the mmap sem.
> The hugetlb path doesn't need to check whether the VMA is anonymous;
> the VM_HUGETLB flag is only set on hugetlbfs VMAs.  There should be no
> performance change from the previous commit; this is simply a step to
> ease bisection of any problems.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

> ---
>  mm/hugetlb.c |  6 ++++++
>  mm/memory.c  | 18 +++++++++---------
>  2 files changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e4a28ce0667f..109e1ff92bc8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6063,6 +6063,12 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>         int need_wait_lock = 0;
>         unsigned long haddr = address & huge_page_mask(h);
>
> +       /* TODO: Handle faults under the VMA lock */
> +       if (flags & FAULT_FLAG_VMA_LOCK) {
> +               vma_end_read(vma);
> +               return VM_FAULT_RETRY;
> +       }
> +
>         /*
>          * Serialize hugepage allocation and instantiation, so that we don't
>          * get spurious allocation failures if two CPUs race to instantiate
> diff --git a/mm/memory.c b/mm/memory.c
> index f2dcc695f54e..6eda5c5f2069 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4998,10 +4998,10 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
>  }
>
>  /*
> - * By the time we get here, we already hold the mm semaphore
> - *
> - * The mmap_lock may have been released depending on flags and our
> - * return value.  See filemap_fault() and __folio_lock_or_retry().
> + * On entry, we hold either the VMA lock or the mmap_lock
> + * (FAULT_FLAG_VMA_LOCK tells you which).  If VM_FAULT_RETRY is set in
> + * the result, the mmap_lock is not held on exit.  See filemap_fault()
> + * and __folio_lock_or_retry().
>   */
>  static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>                 unsigned long address, unsigned int flags)
> @@ -5020,6 +5020,11 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>         p4d_t *p4d;
>         vm_fault_t ret;
>
> +       if ((flags & FAULT_FLAG_VMA_LOCK) && !vma_is_anonymous(vma)) {
> +               vma_end_read(vma);
> +               return VM_FAULT_RETRY;
> +       }
> +
>         pgd = pgd_offset(mm, address);
>         p4d = p4d_alloc(mm, pgd, address);
>         if (!p4d)
> @@ -5247,11 +5252,6 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>                 goto out;
>         }
>
> -       if ((flags & FAULT_FLAG_VMA_LOCK) && !vma_is_anonymous(vma)) {
> -               vma_end_read(vma);
> -               return VM_FAULT_RETRY;
> -       }
> -
>         /*
>          * Enable the memcg OOM handling for faults triggered in user
>          * space.  Kernel faults are handled more gracefully.
> --
> 2.39.2
>





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