Re: [PATCH] block: modify __bio_add_page check to accept pages that don't start a new segment

[Jan Vesely <jvesely@redhat.com>]

Hi Jens,

I have added you to cc, I'm not sure who to bug to get this patch 
merged.

thanks,
Jan Vesely

On Thu 21 Feb 2013 09:30:26 CET, Jan Vesely wrote:
> The original behavior was to refuse all pages after the maximum number of
> segments has been reached. However, some drivers (like st) craft their buffers
> to potentially require exactly max segments and multiple pages in the last
> segment. This patch modifies the check to allow pages that can be merged into
> the last segment.
>
> This change fixes EBUSY failures when using large (1mb) tape block size in high
> memory fragmentation condition.
>
> Signed-off-by: Jan Vesely <jvesely@redhat.com>
> ---
>  fs/bio.c |   26 ++++++++++++++++----------
>  1 files changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/fs/bio.c b/fs/bio.c
> index b96fc6c..02efbd5 100644
> --- a/fs/bio.c
> +++ b/fs/bio.c
> @@ -500,7 +500,6 @@ static int __bio_add_page(struct request_queue *q, struct
> bio *bio, struct page
>                *page, unsigned int len, unsigned int offset,
>                unsigned short max_sectors)
>  {
> -    int retried_segments = 0;
>      struct bio_vec *bvec;
>
>      /*
> @@ -551,18 +550,12 @@ static int __bio_add_page(struct request_queue *q,
> struct bio *bio, struct page
>          return 0;
>
>      /*
> -     * we might lose a segment or two here, but rather that than
> -     * make this too complex.
> +     * prepare segment count check, reduce segment count if possible
>       */
>
> -    while (bio->bi_phys_segments >= queue_max_segments(q)) {
> -
> -        if (retried_segments)
> -            return 0;
> -
> -        retried_segments = 1;
> +    if (bio->bi_phys_segments >= queue_max_segments(q))
>          blk_recount_segments(q, bio);
> -    }
> +
>
>      /*
>       * setup the new entry, we might clear it again later if we
> @@ -572,6 +565,19 @@ static int __bio_add_page(struct request_queue *q, struct
> bio *bio, struct page
>      bvec->bv_page = page;
>      bvec->bv_len = len;
>      bvec->bv_offset = offset;
> +
> +    /*
> +     * the other part of the segment count check, allow mergeable pages
> +     */
> +    if ((bio->bi_phys_segments > queue_max_segments(q)) ||
> +        ( (bio->bi_phys_segments == queue_max_segments(q)) &&
> +        !BIOVEC_PHYS_MERGEABLE(bvec - 1, bvec))) {
> +            bvec->bv_page = NULL;
> +            bvec->bv_len = 0;
> +            bvec->bv_offset = 0;
> +            return 0;
> +    }
> +
>
>      /*
>       * if queue has other restrictions (eg varying max sector size

--
Jan Vesely <jvesely@redhat.com>