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From: Ming Lei <tom.leiming@gmail.com>
Date: Thu, 19 Jul 2018 18:09:50 +0800
Message-ID: <CACVXFVNAZy6YKPt9NFr9KeKvXWY-MP6rQ0=cGFACjo5+gE5pZQ@mail.gmail.com>
Subject: Re: [PATCH 1/2] block: bio_iov_iter_get_pages: fix size of last iovec
To: Martin Wilck <mwilck@suse.com>
Cc: Jens Axboe <axboe@kernel.dk>, Ming Lei <ming.lei@redhat.com>, Jan Kara <jack@suse.com>,
	Hannes Reinecke <hare@suse.de>, Johannes Thumshirn <jthumshirn@suse.de>,
	Kent Overstreet <kent.overstreet@gmail.com>, Christoph Hellwig <hch@lst.de>,
	linux-block <linux-block@vger.kernel.org>
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On Thu, Jul 19, 2018 at 5:39 PM, Martin Wilck <mwilck@suse.com> wrote:
> If the last page of the bio is not "full", the length of the last
> vector slot needs to be corrected. This slot has the index
> (bio->bi_vcnt - 1), but only in bio->bi_io_vec. In the "bv" helper
> array, which is shifted by the value of bio->bi_vcnt at function
> invocation, the correct index is (nr_pages - 1).
>
> V2: improved readability following suggestions from Ming Lei.
>
> Fixes: 2cefe4dbaadf ("block: add bio_iov_iter_get_pages()")
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  block/bio.c | 18 ++++++++----------
>  1 file changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 67eff5e..0964328 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -912,16 +912,16 @@ EXPORT_SYMBOL(bio_add_page);
>   */
>  int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>  {
> -       unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
> +       unsigned short idx, nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
>         struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
>         struct page **pages = (struct page **)bv;
> -       size_t offset, diff;
> +       size_t offset;
>         ssize_t size;
>
>         size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
>         if (unlikely(size <= 0))
>                 return size ? size : -EFAULT;
> -       nr_pages = (size + offset + PAGE_SIZE - 1) / PAGE_SIZE;
> +       idx = nr_pages = (size + offset + PAGE_SIZE - 1) / PAGE_SIZE;
>
>         /*
>          * Deep magic below:  We need to walk the pinned pages backwards
> @@ -934,17 +934,15 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>         bio->bi_iter.bi_size += size;
>         bio->bi_vcnt += nr_pages;
>
> -       diff = (nr_pages * PAGE_SIZE - offset) - size;
> -       while (nr_pages--) {
> -               bv[nr_pages].bv_page = pages[nr_pages];
> -               bv[nr_pages].bv_len = PAGE_SIZE;
> -               bv[nr_pages].bv_offset = 0;
> +       while (idx--) {
> +               bv[idx].bv_page = pages[idx];
> +               bv[idx].bv_len = PAGE_SIZE;
> +               bv[idx].bv_offset = 0;
>         }
>
>         bv[0].bv_offset += offset;
>         bv[0].bv_len -= offset;
> -       if (diff)
> -               bv[bio->bi_vcnt - 1].bv_len -= diff;
> +       bv[nr_pages - 1].bv_len -= nr_pages * PAGE_SIZE - offset - size;
>
>         iov_iter_advance(iter, size);
>         return 0;
> --
> 2.17.1
>

Reviewed-by: Ming Lei <ming.lei@redhat.com>

BTW, we can make it for mainline too, then rebase Christoph's new patch
against this one.

Thanks,
Ming Lei