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.
