linux-kernel - Re: [PATCH v9 8/8] block: convert bio_map_user_iov to use iov_iter_extract

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Message-ID: <df718bc0-2163-a7a7-8c5c-db22e9320b7c@nvidia.com>
Date:   Tue, 24 Jan 2023 12:08:15 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>
CC:     Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>,
        "Jan Kara" <jack@...e.cz>, Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        <linux-fsdevel@...r.kernel.org>, <linux-block@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, "Christoph Hellwig" <hch@....de>
Subject: Re: [PATCH v9 8/8] block: convert bio_map_user_iov to use
 iov_iter_extract_pages

On 1/24/23 09:01, David Howells wrote:
> This will pin pages or leave them unaltered rather than getting a ref on
> them as appropriate to the iterator.
> 
> The pages need to be pinned for DIO rather than having refs taken on them
> to prevent VM copy-on-write from malfunctioning during a concurrent fork()
> (the result of the I/O could otherwise end up being visible to/affected by
> the child process).
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: Al Viro <viro@...iv.linux.org.uk>
> cc: Jens Axboe <axboe@...nel.dk>
> cc: Jan Kara <jack@...e.cz>
> cc: Christoph Hellwig <hch@....de>
> cc: Matthew Wilcox <willy@...radead.org>
> cc: Logan Gunthorpe <logang@...tatee.com>
> cc: linux-block@...r.kernel.org
> ---
> 
> Notes:
>      ver #8)
>       - Split the patch up a bit [hch].
>       - We should only be using pinned/non-pinned pages and not ref'd pages,
>         so adjust the comments appropriately.
>      
>      ver #7)
>       - Don't treat BIO_PAGE_REFFED/PINNED as being the same as FOLL_GET/PIN.
>      
>      ver #5)
>       - Transcribe the FOLL_* flags returned by iov_iter_extract_pages() to
>         BIO_* flags and got rid of bi_cleanup_mode.
>       - Replaced BIO_NO_PAGE_REF to BIO_PAGE_REFFED in the preceding patch.
> 
>   block/blk-map.c | 22 ++++++++++------------
>   1 file changed, 10 insertions(+), 12 deletions(-)
> 

Reviewed-by: John Hubbard <jhubbard@...dia.com>

thanks,
-- 
John Hubbard
NVIDIA

> diff --git a/block/blk-map.c b/block/blk-map.c
> index 0e2b0a861ba3..4e22dccdbe9b 100644
> --- a/block/blk-map.c
> +++ b/block/blk-map.c
> @@ -282,21 +282,19 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
>   	if (blk_queue_pci_p2pdma(rq->q))
>   		extraction_flags |= ITER_ALLOW_P2PDMA;
>   
> -	bio_set_flag(bio, BIO_PAGE_REFFED);
> +	bio_set_cleanup_mode(bio, iter);
>   	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, extraction_flags);
> -		} else {
> -			bytes = iov_iter_get_pages_alloc(iter, &pages,
> -						LONG_MAX, &offs, extraction_flags);
> -		}
> +		if (nr_vecs > ARRAY_SIZE(stack_pages))
> +			pages = NULL;
> +
> +		bytes = iov_iter_extract_pages(iter, &pages, LONG_MAX,
> +					       nr_vecs, extraction_flags, &offs);
>   		if (unlikely(bytes <= 0)) {
>   			ret = bytes ? bytes : -EFAULT;
>   			goto out_unmap;
> @@ -318,7 +316,7 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
>   				if (!bio_add_hw_page(rq->q, bio, page, n, offs,
>   						     max_sectors, &same_page)) {
>   					if (same_page)
> -						put_page(page);
> +						bio_release_page(bio, page);
>   					break;
>   				}
>   
> @@ -330,7 +328,7 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
>   		 * release the pages we didn't map into the bio, if any
>   		 */
>   		while (j < npages)
> -			put_page(pages[j++]);
> +			bio_release_page(bio, pages[j++]);
>   		if (pages != stack_pages)
>   			kvfree(pages);
>   		/* couldn't stuff something into bio? */
>