Re: [PATCH V9 4/7] io_uring: reuse io_mapped_buf for kernel buffer

[Jens Axboe <axboe@kernel.dk>]

On 11/6/24 5:26 AM, Ming Lei wrote:
> Prepare for supporting kernel buffer in case of io group, in which group
> leader leases kernel buffer to io_uring, and consumed by io_uring OPs.
> 
> So reuse io_mapped_buf for group kernel buffer, and unfortunately
> io_import_fixed() can't be reused since userspace fixed buffer is
> virt-contiguous, but it isn't true for kernel buffer.
> 
> Also kernel buffer lifetime is bound with group leader request, it isn't
> necessary to use rsrc_node for tracking its lifetime, especially it needs
> extra allocation of rsrc_node for each IO.

While it isn't strictly necessary, I do think it'd clean up the io_kiocb
parts and hopefully unify the assign and put path more. So I'd strongly
suggest you do use an io_rsrc_node, even if it does just map the
io_mapped_buf for this.

> +struct io_mapped_buf {
> +	u64		start;
> +	unsigned int	len;
> +	unsigned int	nr_bvecs;
> +
> +	/* kbuf hasn't refs and accounting, its lifetime is bound with req */
> +	union {
> +		struct {
> +			refcount_t	refs;
> +			unsigned int	acct_pages;
> +		};
> +		/* pbvec is only for kbuf */
> +		const struct bio_vec	*pbvec;
> +	};
> +	unsigned int	folio_shift:6;
> +	unsigned int	dir:1;		/* ITER_DEST or ITER_SOURCE */
> +	unsigned int	kbuf:1;		/* kernel buffer or not */
> +	/* offset in the 1st bvec, for kbuf only */
> +	unsigned int	offset;
> +	struct bio_vec	bvec[] __counted_by(nr_bvecs);
> +};

And then I'd get rid of this union, and have it follow the normal rules
for an io_mapped_buf in that the refs are valid. Yes it'll take 8b more,
but honestly I think unifying these bits and keeping it consistent is a
LOT more important than saving a bit of space.

This is imho the last piece missing to make this conform more nicely
with how resource nodes are generally handled and used.

-- 
Jens Axboe