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From: Al Viro <viro@zeniv.linux.org.uk>
To: Jens Axboe <axboe@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Pavel Begunkov <asml.silence@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [git pull] iov_iter fixes
Date: Fri, 10 Sep 2021 16:56:28 +0000	[thread overview]
Message-ID: <YTuOPAFvGpayTBpp@zeniv-ca.linux.org.uk> (raw)
In-Reply-To: <1a61c333-680d-71a0-3849-5bfef555a49f@kernel.dk>

On Fri, Sep 10, 2021 at 10:06:25AM -0600, Jens Axboe wrote:

> Looks something like this. Not super pretty in terms of needing a define
> for this, and maybe I'm missing something, but ideally we'd want it as
> an anonymous struct that's defined inside iov_iter. Anyway, gets the
> point across. Alternatively, since we're down to just a few members now,
> we just duplicate them in each struct...
> 
> Would be split into two patches, one for the iov_state addition and
> the save/restore helpers, and then one switching io_uring to use them.
> Figured we'd need some agreement on this first...

> +#define IOV_ITER_STATE					\
> +	size_t iov_offset;				\
> +	size_t count;					\
> +	union {						\
> +		unsigned long nr_segs;			\
> +		struct {				\
> +			unsigned int head;		\
> +			unsigned int start_head;	\
> +		};					\
> +		loff_t xarray_start;			\
> +	};						\
> +
> +struct iov_iter_state {
> +	IOV_ITER_STATE;
> +};
> +
>  struct iov_iter {
>  	u8 iter_type;
>  	bool data_source;
> -	size_t iov_offset;
> -	size_t count;
>  	union {
>  		const struct iovec *iov;
>  		const struct kvec *kvec;
> @@ -40,12 +54,10 @@ struct iov_iter {
>  		struct pipe_inode_info *pipe;
>  	};
>  	union {
> -		unsigned long nr_segs;
> +		struct iov_iter_state state;
>  		struct {
> -			unsigned int head;
> -			unsigned int start_head;
> +			IOV_ITER_STATE;
>  		};
> -		loff_t xarray_start;
>  	};
>  	size_t truncated;
>  };

No.  This is impossible to read *and* wrong for flavours other than
iovec anyway.

Rules:
	count is flavour-independent
	iovec: iov, nr_segs, iov_offset.  nr_segs + iov is constant
	kvec: kvec, nr_segs, iov_offset.  nr_segs + kvec is constant
	bvec: bvec, nr_segs, iov_offset.  nr_segs + bvec is constant
	xarray: xarray, xarray_start, iov_offset.  xarray and xarray_start are constant.
	pipe: pipe, head, start_head, iov_offset.  pipe and start_head are constant,
						   iov_offset can be derived from the rest.
	discard: nothing.

What's more, for pipe (output-only) the situation is much trickier and
there this "reset + advance" won't work at all.  Simply not applicable.

What's the point of all those contortions, anyway?  You only need it for
iovec case; don't mix doing that and turning it into flavour-independent
primitive.

Especially since you turn around and access the fields of that sucker
(->count, that is) directly in your code.  Keep it simple and readable,
please.  We'll sort the sane flavour-independent API later.  And get
rid of ->truncate, while we are at it.

  parent reply	other threads:[~2021-09-10 16:56 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-09  4:22 [git pull] iov_iter fixes Al Viro
2021-09-09 19:37 ` Linus Torvalds
2021-09-09 21:19   ` Jens Axboe
2021-09-09 21:39     ` Jens Axboe
2021-09-09 21:56       ` Linus Torvalds
2021-09-09 22:21         ` Jens Axboe
2021-09-09 22:56           ` Linus Torvalds
2021-09-10  1:35             ` Jens Axboe
2021-09-10  2:43               ` Jens Axboe
2021-09-10  2:48               ` Al Viro
2021-09-10  3:06                 ` Jens Axboe
2021-09-10  3:15                   ` Al Viro
2021-09-10  3:23                     ` Jens Axboe
2021-09-10  3:24                     ` Al Viro
2021-09-10  3:28                       ` Jens Axboe
2021-09-13 15:29                 ` David Laight
2021-09-09 21:42     ` Dave Chinner
2021-09-10  2:57     ` Al Viro
2021-09-10  3:05       ` Jens Axboe
2021-09-10  3:11         ` Al Viro
2021-09-10  3:22           ` Jens Axboe
2021-09-10  3:27             ` Al Viro
2021-09-10  3:30               ` Jens Axboe
2021-09-10  3:36                 ` Al Viro
2021-09-10 13:57                   ` Jens Axboe
2021-09-10 14:42                     ` Al Viro
2021-09-10 15:08                       ` Jens Axboe
2021-09-10 15:32                         ` Al Viro
2021-09-10 15:36                           ` Jens Axboe
2021-09-10 15:04                     ` Jens Axboe
2021-09-10 16:06                       ` Jens Axboe
2021-09-10 16:44                         ` Linus Torvalds
2021-09-10 16:56                         ` Al Viro [this message]
2021-09-10 16:58                           ` Linus Torvalds
2021-09-10 17:26                             ` Jens Axboe
2021-09-10 17:31                               ` Linus Torvalds
2021-09-10 17:32                                 ` Jens Axboe
2021-09-10 18:48                                 ` Al Viro
2021-09-10 19:04                                   ` Linus Torvalds
2021-09-10 19:10                                     ` Linus Torvalds
2021-09-10 19:10                                   ` Jens Axboe
2021-09-10 17:04                           ` Jens Axboe
2021-09-09 22:54   ` Pavel Begunkov
2021-09-09 22:57     ` Pavel Begunkov
2021-09-09 23:14   ` Pavel Begunkov
2021-09-09 20:03 ` pr-tracker-bot

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