Re: [PATCHSET v3 0/3] Add ability to save/restore iov_iter state

[Jens Axboe <axboe@kernel.dk>]

On 9/15/21 10:47 PM, Al Viro wrote:
> 	Jens, may I politely inquire why is struct io_rw playing
> these games with overloading ->rw.addr, instead of simply having
> struct io_buffer *kbuf in it?

Very simply to avoid growing the union command part of io_kiocb beyond a
cacheline. We're pretty sensitive to io_kiocb size in general, and io_rw
is already the biggest member in there.
 
> 	Another question: what the hell are the rules for
> REQ_F_BUFFER_SELECT?  The first time around io_iov_buffer_select()
> will
> 	* read iovec from ->rw.addr
> 	* replace iovec.iov_base with value derived from
> ->buf_index
> 	* cap iovec.iov_len with value derived from ->buf_index
> Next time around it will use the same base *AND* replace the
> length with the value used to cap the original.
> 	Is that deliberate?

Probably not strictly needed, but doesn't harm anything. The buffer is
being consumed (and hence removed) at completion anyway, it's not a
persistent change. Selected buffers must be re-provided by the
application as the kernel has no way of knowing when the application
would otherwise be ready for it to get reused, and that's done by
issuing a new provide buffers request for the buffers that can get
recycled.

-- 
Jens Axboe