Re: [PATCH v2 2/6] mm: Call wp_page_copy() under the VMA lock

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Oct 6, 2023 at 12:53 PM Matthew Wilcox (Oracle)
<willy@infradead.org> wrote:
>
> It is usually safe to call wp_page_copy() under the VMA lock.  The only
> unsafe situation is when no anon_vma has been allocated for this VMA,
> and we have to look at adjacent VMAs to determine if their anon_vma can
> be shared.  Since this happens only for the first COW of a page in this
> VMA, the majority of calls to wp_page_copy() do not need to fall back
> to the mmap_sem.
>
> Add vmf_anon_prepare() as an alternative to anon_vma_prepare() which
> will return RETRY if we currently hold the VMA lock and need to allocate
> an anon_vma.  This lets us drop the check in do_wp_page().
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  mm/memory.c | 39 ++++++++++++++++++++++++++-------------
>  1 file changed, 26 insertions(+), 13 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 97f860d6cd2a..cff78c496728 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3042,6 +3042,21 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
>         count_vm_event(PGREUSE);
>  }
>
> +static vm_fault_t vmf_anon_prepare(struct vm_fault *vmf)
> +{
> +       struct vm_area_struct *vma = vmf->vma;
> +
> +       if (likely(vma->anon_vma))
> +               return 0;
> +       if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> +               vma_end_read(vma);
> +               return VM_FAULT_RETRY;
> +       }
> +       if (__anon_vma_prepare(vma))
> +               return VM_FAULT_OOM;
> +       return 0;
> +}
> +
>  /*
>   * Handle the case of a page which we actually need to copy to a new page,
>   * either due to COW or unsharing.
> @@ -3069,27 +3084,29 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>         pte_t entry;
>         int page_copied = 0;
>         struct mmu_notifier_range range;
> -       int ret;
> +       vm_fault_t ret;
>
>         delayacct_wpcopy_start();
>
>         if (vmf->page)
>                 old_folio = page_folio(vmf->page);
> -       if (unlikely(anon_vma_prepare(vma)))
> -               goto oom;
> +       ret = vmf_anon_prepare(vmf);
> +       if (unlikely(ret))
> +               goto out;
>
>         if (is_zero_pfn(pte_pfn(vmf->orig_pte))) {
>                 new_folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
>                 if (!new_folio)
>                         goto oom;
>         } else {
> +               int err;
>                 new_folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma,
>                                 vmf->address, false);
>                 if (!new_folio)
>                         goto oom;
>
> -               ret = __wp_page_copy_user(&new_folio->page, vmf->page, vmf);
> -               if (ret) {
> +               err = __wp_page_copy_user(&new_folio->page, vmf->page, vmf);
> +               if (err) {
>                         /*
>                          * COW failed, if the fault was solved by other,
>                          * it's fine. If not, userspace would re-fault on
> @@ -3102,7 +3119,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>                                 folio_put(old_folio);
>
>                         delayacct_wpcopy_end();
> -                       return ret == -EHWPOISON ? VM_FAULT_HWPOISON : 0;
> +                       return err == -EHWPOISON ? VM_FAULT_HWPOISON : 0;
>                 }
>                 kmsan_copy_page_meta(&new_folio->page, vmf->page);
>         }
> @@ -3212,11 +3229,13 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>  oom_free_new:
>         folio_put(new_folio);
>  oom:
> +       ret = VM_FAULT_OOM;
> +out:
>         if (old_folio)
>                 folio_put(old_folio);
>
>         delayacct_wpcopy_end();
> -       return VM_FAULT_OOM;
> +       return ret;
>  }
>
>  /**
> @@ -3458,12 +3477,6 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>                 return 0;
>         }
>  copy:
> -       if ((vmf->flags & FAULT_FLAG_VMA_LOCK) && !vma->anon_vma) {
> -               pte_unmap_unlock(vmf->pte, vmf->ptl);
> -               vma_end_read(vmf->vma);
> -               return VM_FAULT_RETRY;
> -       }
> -
>         /*
>          * Ok, we need to copy. Oh, well..
>          */
> --
> 2.40.1
>