linux-kernel - Re: [RFC PATCH] iov_iter: Add an iterator-of-iterators

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Message-ID: <ZBtNtWKIejBGlKVv@casper.infradead.org>
Date:   Wed, 22 Mar 2023 18:49:25 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Jeffrey Layton <jlayton@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Anna Schumaker <anna@...nel.org>,
        Charles Edward Lever <chuck.lever@...cle.com>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: [RFC PATCH] iov_iter: Add an iterator-of-iterators

On Wed, Mar 22, 2023 at 06:15:45PM +0000, David Howells wrote:
> @@ -43,17 +44,17 @@ struct iov_iter {
>  	bool nofault;
>  	bool data_source;
>  	bool user_backed;
> -	union {
> -		size_t iov_offset;
> -		int last_offset;
> -	};
> +	bool spliceable;

We've now up to five u8s in a row here (iter_type, nofault, data_source,
user_backed).  Is it time to turn some/all of them into:

	bool nofault:1;
	bool data_source:1;
	bool user_backed:1;
	bool spliceable:1;

You can't take the address of them then, but I don't believe we do that
anywhere.