Re: [RFC PATCH v4 4/5] btrfs: ioctl: introduce btrfs_uring_import_iovec()

[Sidong Yang <sidong.yang@furiosa.ai>]

On Tue, Mar 18, 2025 at 07:25:51AM +0000, Pavel Begunkov wrote:
> On 3/18/25 01:58, Sidong Yang wrote:
> > On Mon, Mar 17, 2025 at 09:40:05AM -0600, Jens Axboe wrote:
> > > On 3/17/25 7:57 AM, Sidong Yang wrote:
> > > > This patch introduces btrfs_uring_import_iovec(). In encoded read/write
> > > > with uring cmd, it uses import_iovec without supporting fixed buffer.
> > > > btrfs_using_import_iovec() could use fixed buffer if cmd flags has
> > > > IORING_URING_CMD_FIXED.
> > > > 
> > > > Signed-off-by: Sidong Yang <sidong.yang@furiosa.ai>
> > > > ---
> > > >   fs/btrfs/ioctl.c | 32 ++++++++++++++++++++++++--------
> > > >   1 file changed, 24 insertions(+), 8 deletions(-)
> > > > 
> > > > diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> > > > index 6c18bad53cd3..a7b52fd99059 100644
> > > > --- a/fs/btrfs/ioctl.c
> > > > +++ b/fs/btrfs/ioctl.c
> > > > @@ -4802,6 +4802,28 @@ struct btrfs_uring_encoded_data {
> > > >   	struct iov_iter iter;
> > > >   };
> > > > +static int btrfs_uring_import_iovec(struct io_uring_cmd *cmd,
> > > > +				    unsigned int issue_flags, int rw)
> > > > +{
> > > > +	struct btrfs_uring_encoded_data *data =
> > > > +		io_uring_cmd_get_async_data(cmd)->op_data;
> > > > +	int ret;
> > > > +
> > > > +	if (cmd && (cmd->flags & IORING_URING_CMD_FIXED)) {
> > > > +		data->iov = NULL;
> > > > +		ret = io_uring_cmd_import_fixed_vec(cmd, data->args.iov,
> > > > +						    data->args.iovcnt,
> > > > +						    ITER_DEST, issue_flags,
> > > > +						    &data->iter);
> > > > +	} else {
> > > > +		data->iov = data->iovstack;
> > > > +		ret = import_iovec(rw, data->args.iov, data->args.iovcnt,
> > > > +				   ARRAY_SIZE(data->iovstack), &data->iov,
> > > > +				   &data->iter);
> > > > +	}
> > > > +	return ret;
> > > > +}
> > > 
> > > How can 'cmd' be NULL here?
> > 
> > It seems that there is no checkes for 'cmd' before and it works same as before.
> > But I think it's better to add a check in function start for safety.
> 
> The check goes two lines after you already dereferenced it, and it
> seems to be called from io_uring cmd specific code. The null check
> only adds to confusion.

You mean 'cmd' already dereferenced for async_data. Is it okay to just delete code
checking 'cmd' like below?

if (cmd->flags & IORING_URING_CMD_FIXED) {

Thanks,
Sidong

> 
> -- 
> Pavel Begunkov
>