Re: [RFC PATCH 3/3] ovl: implement stacked mmap for shared map

[Amir Goldstein <amir73il@gmail.com>]

On Sat, Aug 29, 2020 at 12:51 PM Chengguang Xu <cgxu519@mykernel.net> wrote:
>
> Implement stacked mmap for shared map to keep data
> consistency.
>
> Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
> ---
>  fs/overlayfs/file.c | 120 +++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 114 insertions(+), 6 deletions(-)
>
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index 14ab5344a918..db5ab200d984 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -21,9 +21,17 @@ struct ovl_aio_req {
>         struct fd fd;
>  };
>
> +static vm_fault_t ovl_fault(struct vm_fault *vmf);
> +static vm_fault_t ovl_page_mkwrite(struct vm_fault *vmf);
> +
> +static const struct vm_operations_struct ovl_vm_ops = {
> +       .fault          = ovl_fault,
> +       .page_mkwrite   = ovl_page_mkwrite,
> +};
> +

Interesting direction, not sure if this is workable.
I don't know enough about mm to say.

But what about the rest of the operations?
Did you go over them and decide that overlay doesn't need to implement them?
I doubt it, but if you did, please document that.

>  struct ovl_file_entry {
>         struct file *realfile;
> -       void *vm_ops;
> +       const struct vm_operations_struct *vm_ops;
>  };
>
>  struct file *ovl_get_realfile(struct file *file)
> @@ -40,14 +48,15 @@ void ovl_set_realfile(struct file *file, struct file *realfile)
>         ofe->realfile = realfile;
>  }
>
> -void *ovl_get_real_vmops(struct file *file)
> +const struct vm_operations_struct *ovl_get_real_vmops(struct file *file)
>  {
>         struct ovl_file_entry *ofe = file->private_data;
>
>         return ofe->vm_ops;
>  }
>
> -void ovl_set_real_vmops(struct file *file, void *vm_ops)
> +void ovl_set_real_vmops(struct file *file,
> +                       const struct vm_operations_struct *vm_ops)
>  {
>         struct ovl_file_entry *ofe = file->private_data;
>
> @@ -493,11 +502,104 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
>         return ret;
>  }
>
> +vm_fault_t ovl_fault(struct vm_fault *vmf)
> +{
> +       struct vm_area_struct *vma = vmf->vma;
> +       struct file *file = vma->vm_file;
> +       struct file *realfile;
> +       struct file *fpin, *tmp;
> +       struct inode *inode = file_inode(file);
> +       struct inode *realinode;
> +       const struct cred *old_cred;
> +       bool retry_allowed;
> +       vm_fault_t ret;
> +       int err = 0;
> +
> +       if (fault_flag_check(vmf, FAULT_FLAG_TRIED)) {
> +               realfile = ovl_get_realfile(file);
> +
> +               if (!ovl_has_upperdata(inode) ||
> +                   realfile->f_inode != ovl_inode_upper(inode) ||
> +                   !realfile->f_op->mmap)
> +                       return VM_FAULT_SIGBUS;
> +
> +               if (!ovl_get_real_vmops(file)) {
> +                       old_cred = ovl_override_creds(inode->i_sb);
> +                       err = call_mmap(realfile, vma);
> +                       revert_creds(old_cred);
> +
> +                       vma->vm_file = file;
> +                       if (err) {
> +                               vma->vm_ops = &ovl_vm_ops;
> +                               return VM_FAULT_SIGBUS;
> +                       }
> +                       ovl_set_real_vmops(file, vma->vm_ops);
> +                       vma->vm_ops = &ovl_vm_ops;
> +               }
> +
> +               retry_allowed = fault_flag_check(vmf, FAULT_FLAG_ALLOW_RETRY);
> +               if (retry_allowed)
> +                       vma->vm_flags &= ~FAULT_FLAG_ALLOW_RETRY;
> +               vma->vm_file = realfile;
> +               ret = ovl_get_real_vmops(file)->fault(vmf);
> +               vma->vm_file = file;
> +               if (retry_allowed)
> +                       vma->vm_flags |= FAULT_FLAG_ALLOW_RETRY;
> +               return ret;
> +
> +       } else {
> +               fpin = maybe_unlock_mmap_for_io(vmf, NULL);
> +               if (!fpin)
> +                       return VM_FAULT_SIGBUS;
> +
> +               ret = VM_FAULT_RETRY;
> +               if (!ovl_has_upperdata(inode)) {
> +                       err = ovl_copy_up_with_data(file->f_path.dentry);
> +                       if (err)
> +                               goto out;
> +               }
> +
> +               realinode = ovl_inode_realdata(inode);
> +               realfile = ovl_open_realfile(file, realinode);
> +               if (IS_ERR(realfile))
> +                       goto out;
> +
> +               tmp = ovl_get_realfile(file);
> +               ovl_set_realfile(file, realfile);
> +               fput(tmp);
> +
> +out:
> +               fput(fpin);
> +               return ret;
> +       }
> +}


Please add some documentation to explain the method used.
Do we need to retry if real_vmops are already set?

Thanks,
Amir.