linux-kernel - Re: [RFC][PATCH] iov_iter: Add ITER

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Date:   Wed, 22 Jan 2020 11:33:06 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] iov_iter: Add ITER_MAPPING

On Wed, Jan 22, 2020 at 02:57:55PM +0000, David Howells wrote:
> An alternative could be to have an "ITER_ARRAY" that just has the page
> pointers and not the offset/length info.  This decreases the redundancy and
> increases the max payload-per-array-page to 2M.

We could also have an ITER_XARRAY which you just pass &mapping->i_pages
to.  I don't think you use any other part of the mapping, so that would
be a more generic version that is equally efficient.

> +	rcu_read_lock();						\
> +	for (page = xas_load(&xas); page; page = xas_next(&xas)) {	\
> +		if (xas_retry(&xas, page))				\
> +			continue;					\
> +		if (xa_is_value(page))					\
> +			break;						\

Do you also want to check for !page?  That would be a bug in the caller.

> +		if (PageCompound(page))					\
> +			break;						\

It's perfectly legal to have compound pages in the page cache.  Call
find_subpage(page, xas.xa_index) unconditionally.

> +		if (page_to_pgoff(page) != xas.xa_index)		\
> +			break;						\

... and you can ditch this if the pages are pinned as find_subpage()
will bug in this case.

> +		__v.bv_page = page;					\
> +		offset = (i->mapping_start + skip) & ~PAGE_MASK;	\
> +		seg = PAGE_SIZE - offset;			\
> +		__v.bv_offset = offset;				\
> +		__v.bv_len = min(n, seg);			\
> +		(void)(STEP);					\
> +		n -= __v.bv_len;				\
> +		skip += __v.bv_len;				\

Do we want STEP to be called with PAGE_SIZE chunks, or if they have a
THP, can we have it called with larger than a PAGE_SIZE chunk?

> +#define iterate_all_kinds(i, n, v, I, B, K, M) {		\
>  	if (likely(n)) {					\
>  		size_t skip = i->iov_offset;			\
>  		if (unlikely(i->type & ITER_BVEC)) {		\
> @@ -86,6 +119,9 @@
>  			struct kvec v;				\
>  			iterate_kvec(i, n, v, kvec, skip, (K))	\
>  		} else if (unlikely(i->type & ITER_DISCARD)) {	\
> +		} else if (unlikely(i->type & ITER_MAPPING)) {	\
> +			struct bio_vec v;			\
> +			iterate_mapping(i, n, v, skip, (M));	\

bio_vec?

> -#define iterate_and_advance(i, n, v, I, B, K) {			\
> +#define iterate_and_advance(i, n, v, I, B, K, M) {		\
>  	if (unlikely(i->count < n))				\
>  		n = i->count;					\
>  	if (i->count) {						\
> @@ -119,6 +155,9 @@
>  			i->kvec = kvec;				\
>  		} else if (unlikely(i->type & ITER_DISCARD)) {	\
>  			skip += n;				\
> +		} else if (unlikely(i->type & ITER_MAPPING)) {	\
> +			struct bio_vec v;			\
> +			iterate_mapping(i, n, v, skip, (M))	\

again?