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Date:   Tue, 24 Jan 2023 12:00:16 -0800
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Subject: Re: [PATCH v9 7/8] block: Convert bio_iov_iter_get_pages to use
 iov_iter_extract_pages
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To:     David Howells <dhowells@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>
CC:     Matthew Wilcox <willy@infradead.org>, Jens Axboe <axboe@kernel.dk>,
        "Jan Kara" <jack@suse.cz>, Jeff Layton <jlayton@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        <linux-fsdevel@vger.kernel.org>, <linux-block@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, "Christoph Hellwig" <hch@lst.de>
References: <20230124170108.1070389-1-dhowells@redhat.com>
 <20230124170108.1070389-8-dhowells@redhat.com>
From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <20230124170108.1070389-8-dhowells@redhat.com>
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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 affected by/visible to the
> child process).
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Al Viro <viro@zeniv.linux.org.uk>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: Jan Kara <jack@suse.cz>
> cc: Christoph Hellwig <hch@lst.de>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: Logan Gunthorpe <logang@deltatee.com>
> cc: linux-block@vger.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/bio.c | 22 +++++++++++-----------
>   1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index fc45aaa97696..936301519e6c 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1212,7 +1212,7 @@ static int bio_iov_add_page(struct bio *bio, struct page *page,
>   	}
>   
>   	if (same_page)
> -		put_page(page);
> +		bio_release_page(bio, page);
>   	return 0;
>   }
>   
> @@ -1226,7 +1226,7 @@ static int bio_iov_add_zone_append_page(struct bio *bio, struct page *page,
>   			queue_max_zone_append_sectors(q), &same_page) != len)
>   		return -EINVAL;
>   	if (same_page)
> -		put_page(page);
> +		bio_release_page(bio, page);
>   	return 0;
>   }
>   
> @@ -1237,10 +1237,10 @@ static int bio_iov_add_zone_append_page(struct bio *bio, struct page *page,
>    * @bio: bio to add pages to
>    * @iter: iov iterator describing the region to be mapped
>    *
> - * Pins pages from *iter and appends them to @bio's bvec array. The
> - * pages will have to be released using put_page() when done.
> - * For multi-segment *iter, this function only adds pages from the
> - * next non-empty segment of the iov iterator.
> + * Extracts pages from *iter and appends them to @bio's bvec array.  The pages
> + * will have to be cleaned up in the way indicated by the BIO_PAGE_PINNED flag.
> + * For a multi-segment *iter, this function only adds pages from the next
> + * non-empty segment of the iov iterator.
>    */
>   static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>   {
> @@ -1272,9 +1272,9 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>   	 * result to ensure the bio's total size is correct. The remainder of
>   	 * the iov data will be picked up in the next bio iteration.
>   	 */
> -	size = iov_iter_get_pages(iter, pages,
> -				  UINT_MAX - bio->bi_iter.bi_size,
> -				  nr_pages, &offset, extraction_flags);
> +	size = iov_iter_extract_pages(iter, &pages,
> +				      UINT_MAX - bio->bi_iter.bi_size,
> +				      nr_pages, extraction_flags, &offset);

A quite minor point: it seems like the last two args got reversed more
or less by accident. It's not worth re-spinning or anything, but it
seems better to leave the order the same between these two routines.

Either way, though,

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

thanks,
-- 
John Hubbard
NVIDIA

>   	if (unlikely(size <= 0))
>   		return size ? size : -EFAULT;
>   
> @@ -1307,7 +1307,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>   	iov_iter_revert(iter, left);
>   out:
>   	while (i < nr_pages)
> -		put_page(pages[i++]);
> +		bio_release_page(bio, pages[i++]);
>   
>   	return ret;
>   }
> @@ -1342,7 +1342,7 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>   		return 0;
>   	}
>   
> -	bio_set_flag(bio, BIO_PAGE_REFFED);
> +	bio_set_cleanup_mode(bio, iter);
>   	do {
>   		ret = __bio_iov_iter_get_pages(bio, iter);
>   	} while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));
>