linux-kernel - Re: [PATCH 1/2] iov: introduce ITER_BVEC_FLAG

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Date:   Wed, 9 Dec 2020 08:36:45 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] iov: introduce ITER_BVEC_FLAG_FIXED

Ok, seems like the patches made it to the lists, while oyu only
send the cover letter to my address which is very strange.

> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index 72d88566694e..af626eb970cf 100644
> --- a/include/linux/uio.h
> +++ b/include/linux/uio.h
> @@ -18,6 +18,8 @@ struct kvec {
>  };
>  
>  enum iter_type {
> +	ITER_BVEC_FLAG_FIXED = 2,
> +
>  	/* iter types */
>  	ITER_IOVEC = 4,
>  	ITER_KVEC = 8,

This is making the iter type even more of a mess than it already is.
I think we at least need placeholders for 0/1 here and an explicit
flags namespace, preferably after the types.

Then again I'd much prefer if we didn't even add the flag or at best
just add it for a short-term transition and move everyone over to the
new scheme.  Otherwise the amount of different interfaces and supporting
code keeps exploding.

> @@ -29,8 +31,9 @@ enum iter_type {
>  struct iov_iter {
>  	/*
>  	 * Bit 0 is the read/write bit, set if we're writing.
> -	 * Bit 1 is the BVEC_FLAG_NO_REF bit, set if type is a bvec and
> -	 * the caller isn't expecting to drop a page reference when done.
> +	 * Bit 1 is the BVEC_FLAG_FIXED bit, set if type is a bvec and the
> +	 * caller ensures that page references and memory baking bvec won't
> +	 * go away until callees finish with them.
>  	 */
>  	unsigned int type;

I think the comment needs to move to the enum.