linux-kernel - Re: [PATCH v6 01/34] vfs: Unconditionally set IOCB_WRITE in call_write

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Message-ID: <Y8ZTyx7vM8NpnUAj@infradead.org>
Date:   Mon, 16 Jan 2023 23:52:43 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, Christoph Hellwig <hch@....de>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 01/34] vfs: Unconditionally set IOCB_WRITE in
 call_write_iter()

On Mon, Jan 16, 2023 at 11:08:09PM +0000, David Howells wrote:
> IOCB_WRITE is set by aio, io_uring and cachefiles before submitting a write
> operation to the VFS, but it isn't set by, say, the write() system call.
> 
> Fix this by setting IOCB_WRITE unconditionally in call_write_iter().
> 
> This will allow drivers to use IOCB_WRITE instead of the iterator data
> source to determine the I/O direction.
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: Alexander Viro <viro@...iv.linux.org.uk>
> cc: Christoph Hellwig <hch@....de>
> cc: Jens Axboe <axboe@...nel.dk>
> cc: linux-block@...r.kernel.org
> cc: linux-fsdevel@...r.kernel.org
> ---
> 
>  include/linux/fs.h |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 066555ad1bf8..649ff061440e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2183,6 +2183,7 @@ static inline ssize_t call_read_iter(struct file *file, struct kiocb *kio,
>  static inline ssize_t call_write_iter(struct file *file, struct kiocb *kio,
>  				      struct iov_iter *iter)
>  {
> +	kio->ki_flags |= IOCB_WRITE;
>  	return file->f_op->write_iter(kio, iter);
>  }

This doesn't remove the existing setting of IOCB_WRITE, and also
feelds like the wrong place.

I suspect the best is to:

 - rename init_sync_kiocb to init_kiocb
 - pass a new argument for the destination to it.  I'm not entirely
   sure if flags is a good thing, or an explicit READ/WRITE might be
   better because it's harder to get wrong, even if a the compiler
   might generate worth code for it.
 - also use it in the async callers (io_uring, aio, overlayfs, loop,
   nvmet, target, cachefs, file backed swap)