linux-kernel - Re: [PATCH 07/12] dm: use bvec iterator helpers to implement .get_page and .next

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Message-ID: <CACVXFVPbG6Dz2F=tc9o=jPdNCcXWDmtReaWGWQeTPa9HaGjh0A@mail.gmail.com>
Date:   Tue, 15 Nov 2016 09:01:53 +0800
From:   Ming Lei <tom.leiming@...il.com>
To:     Jens Axboe <axboe@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     linux-block <linux-block@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Ming Lei <tom.leiming@...il.com>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        "maintainer:DEVICE-MAPPER (LVM)" <dm-devel@...hat.com>,
        Shaohua Li <shli@...nel.org>,
        "open list:SOFTWARE RAID (Multiple Disks) SUPPORT" 
        <linux-raid@...r.kernel.org>
Subject: Re: [PATCH 07/12] dm: use bvec iterator helpers to implement
 .get_page and .next_page

On Fri, Nov 11, 2016 at 8:05 PM, Ming Lei <tom.leiming@...il.com> wrote:
> Firstly we have mature bvec/bio iterator helper for iterate each
> page in one bio, not necessary to reinvent a wheel to do that.
>
> Secondly the coming multipage bvecs requires this patch.
>
> Also add comments about the direct access to bvec table.
>
> Signed-off-by: Ming Lei <tom.leiming@...il.com>
> ---
>  drivers/md/dm-io.c | 34 ++++++++++++++++++++++++----------
>  1 file changed, 24 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/md/dm-io.c b/drivers/md/dm-io.c
> index 0bf1a12e35fe..2ef573c220fc 100644
> --- a/drivers/md/dm-io.c
> +++ b/drivers/md/dm-io.c
> @@ -162,7 +162,10 @@ struct dpages {
>                          struct page **p, unsigned long *len, unsigned *offset);
>         void (*next_page)(struct dpages *dp);
>
> -       unsigned context_u;
> +       union {
> +               unsigned context_u;
> +               struct bvec_iter context_bi;
> +       };
>         void *context_ptr;
>
>         void *vma_invalidate_address;
> @@ -204,25 +207,36 @@ static void list_dp_init(struct dpages *dp, struct page_list *pl, unsigned offse
>  static void bio_get_page(struct dpages *dp, struct page **p,
>                          unsigned long *len, unsigned *offset)
>  {
> -       struct bio_vec *bvec = dp->context_ptr;
> -       *p = bvec->bv_page;
> -       *len = bvec->bv_len - dp->context_u;
> -       *offset = bvec->bv_offset + dp->context_u;
> +       struct bio_vec bv = bvec_iter_bvec((struct bio_vec *)dp->context_ptr,
> +                       dp->context_bi);
> +
> +       *p = bv.bv_page;
> +       *len = bv.bv_len;
> +       *offset = bv.bv_offset;
> +
> +       /* avoid to figure out it in bio_next_page() again */
> +       dp->context_bi.bi_sector = (sector_t)bv.bv_len;
>  }
>
>  static void bio_next_page(struct dpages *dp)
>  {
> -       struct bio_vec *bvec = dp->context_ptr;
> -       dp->context_ptr = bvec + 1;
> -       dp->context_u = 0;
> +       unsigned int len = (unsigned int)dp->context_bi.bi_sector;
> +
> +       bvec_iter_advance((struct bio_vec *)dp->context_ptr,
> +                       &dp->context_bi, len);
>  }
>
>  static void bio_dp_init(struct dpages *dp, struct bio *bio)
>  {
>         dp->get_page = bio_get_page;
>         dp->next_page = bio_next_page;
> -       dp->context_ptr = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
> -       dp->context_u = bio->bi_iter.bi_bvec_done;
> +
> +       /*
> +        * We just use bvec iterator to retrieve pages, so it is ok to
> +        * access the bvec table directly here
> +        */
> +       dp->context_ptr = bio->bi_io_vec;
> +       dp->context_bi = bio->bi_iter;
>  }

Hi Alasdair, Mike, Christoph and anyone,

Could you give this one a review?

>From my test, it just works fine, and I believe it is a good cleanup.

Thanks,
Ming Lei