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.
next prev 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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