On 9/10/21 10:56 AM, Al Viro wrote:
> 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.
Yes that's a good point, BVEC as well fwiw. But those two are very
similar.
> 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.
Alright, so how about I just make the state a bit dumber and only work
for iovec/bvec. That gets rid of the weirdo macro. Add a WARN_ON_ONCE()
for using restore on anything that isn't an IOVEC/BVEC.
Sound reasonable?
--
Jens Axboe
