linux-kernel - Re: [PATCH v6 11/34] iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate

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Date:   Tue, 17 Jan 2023 00:07:28 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, Jens Axboe <axboe@...nel.dk>,
        Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 11/34] iov_iter, block: Make bio structs pin pages
 rather than ref'ing if appropriate

> +	size = iov_iter_extract_pages(iter, &pages,
> +				      UINT_MAX - bio->bi_iter.bi_size,
> +				      nr_pages, gup_flags, &offset);
>  	if (unlikely(size <= 0))


> +	bio_set_cleanup_mode(bio, iter, gup_flags);

This should move out to bio_iov_iter_get_pages and only be called
once.

> +++ b/block/blk-map.c
> @@ -285,24 +285,24 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
>  		gup_flags |= FOLL_PCI_P2PDMA;
>  
>  	while (iov_iter_count(iter)) {
> -		struct page **pages, *stack_pages[UIO_FASTIOV];
> +		struct page *stack_pages[UIO_FASTIOV];
> +		struct page **pages = stack_pages;
>  		ssize_t bytes;
>  		size_t offs;
>  		int npages;
>  
> -		if (nr_vecs <= ARRAY_SIZE(stack_pages)) {
> -			pages = stack_pages;
> -			bytes = iov_iter_get_pages(iter, pages, LONG_MAX,
> -						   nr_vecs, &offs, gup_flags);
> -		} else {
> -			bytes = iov_iter_get_pages_alloc(iter, &pages,
> -						LONG_MAX, &offs, gup_flags);
> -		}
> +		if (nr_vecs > ARRAY_SIZE(stack_pages))
> +			pages = NULL;
> +
> +		bytes = iov_iter_extract_pages(iter, &pages, LONG_MAX,
> +					       nr_vecs, gup_flags, &offs);
>  		if (unlikely(bytes <= 0)) {
>  			ret = bytes ? bytes : -EFAULT;
>  			goto out_unmap;
>  		}
>  
> +		bio_set_cleanup_mode(bio, iter, gup_flags);

Same here - one call outside of the loop.

> +static inline void bio_set_cleanup_mode(struct bio *bio, struct iov_iter *iter,
> +					unsigned int gup_flags)
> +{
> +	unsigned int cleanup_mode;
> +
> +	bio_clear_flag(bio, BIO_PAGE_REFFED);

.. and this should not be needed.  Instead:

> +	cleanup_mode = iov_iter_extract_mode(iter, gup_flags);
> +	if (cleanup_mode & FOLL_GET)
> +		bio_set_flag(bio, BIO_PAGE_REFFED);
> +	if (cleanup_mode & FOLL_PIN)
> +		bio_set_flag(bio, BIO_PAGE_PINNED);

We could warn if a not match flag is set here if we really care.