linux-kernel - Re: [PATCH v16 03/13] overlayfs: Implement splice-read

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Date:   Wed, 8 Mar 2023 16:33:37 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     David Howells <dhowells@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        linux-unionfs@...r.kernel.org
Subject: Re: [PATCH v16 03/13] overlayfs: Implement splice-read

On Wed, 8 Mar 2023 at 15:38, David Howells <dhowells@...hat.com> wrote:
>
> Implement splice-read for overlayfs by passing the request down a layer
> rather than going through generic_file_splice_read() which is going to be
> changed to assume that ->read_folio() is present on buffered files.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: Christoph Hellwig <hch@....de>
> cc: Jens Axboe <axboe@...nel.dk>
> cc: Al Viro <viro@...iv.linux.org.uk>
> cc: John Hubbard <jhubbard@...dia.com>
> cc: David Hildenbrand <david@...hat.com>
> cc: Matthew Wilcox <willy@...radead.org>
> cc: Miklos Szeredi <miklos@...redi.hu>
> cc: linux-unionfs@...r.kernel.org
> cc: linux-block@...r.kernel.org
> cc: linux-fsdevel@...r.kernel.org
> cc: linux-mm@...ck.org
> ---
>
> Notes:
>     ver #15)
>      - Remove redundant FMODE_CAN_ODIRECT check on real file.
>      - Do rw_verify_area() on the real file, not the overlay file.
>      - Fix a file leak.
>
>  fs/overlayfs/file.c | 33 ++++++++++++++++++++++++++++++++-
>  1 file changed, 32 insertions(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index 7c04f033aadd..a12919e9ccba 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -419,6 +419,37 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
>         return ret;
>  }
>
> +static ssize_t ovl_splice_read(struct file *in, loff_t *ppos,
> +                              struct pipe_inode_info *pipe, size_t len,
> +                              unsigned int flags)
> +{
> +       const struct cred *old_cred;
> +       struct fd real;
> +       ssize_t ret;
> +
> +       ret = ovl_real_fdget(in, &real);
> +       if (ret)
> +               return ret;
> +
> +       ret = -EINVAL;
> +       if (!real.file->f_op->splice_read)
> +               goto out_fdput;
> +
> +       ret = rw_verify_area(READ, real.file, ppos, len);
> +       if (unlikely(ret < 0))
> +               goto out_fdput;
> +
> +       old_cred = ovl_override_creds(file_inode(in)->i_sb);
> +       ret = real.file->f_op->splice_read(real.file, ppos, pipe, len, flags);

I don't think you replied to my suggestion of using a helper here.
E.g. it could be as simple as exporting do_splice_to(), or renaming it
to vfs_splice_read() to be more readable.  It would remove the
boilerplate and be more robust if any changes are done to the splice
reading code.

Thanks,
Miklos