Re: [PATCH RESEND v2] block: modify __bio_add_page check to acceptpages that don't start a new segment

[Jens Axboe]

On Mon, Mar 25 2013, Jan Vesely wrote:
> v2: changed a comment
> 
> 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.
> 
> Fixes EBUSY failures when using large tape block size in high
> memory fragmentation condition.
> This regression was introduced by commit
>  46081b166415acb66d4b3150ecefcd9460bb48a1
>  st: Increase success probability in driver buffer allocation
> 
> Signed-off-by: Jan Vesely <jvesely@xxxxxxxxxx>
> 
> CC: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> CC: FUJITA Tomonori <fujita.tomonori@xxxxxxxxxxxxx>
> CC: Kai Makisara <kai.makisara@xxxxxxxxxxx>
> CC: James Bottomley <james.bottomley@xxxxxxxxxxxxxxxxxxxxx>
> CC: Jens Axboe <axboe@xxxxxxxxx>
> CC: stable@xxxxxxxxxxxxxxx
> ---
>  fs/bio.c | 27 +++++++++++++++++----------
>  1 file changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/bio.c b/fs/bio.c
> index bb5768f..bc6af71 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 +550,13 @@ 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.
> +	 * The first part of the 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 +566,19 @@ 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;
> +	}
> +

This is a bit messy, I think. bi_phys_segments should never be allowed
to go beyond queue_ma_segments(), so the > test does not look right.
Maybe it's an artifact of when we fall through with this patch, we bump
bi_phys_segments even if the segments are physicall contig and
mergeable.

What happens when the segment is physically mergeable, but the resulting
merged segment is too large (bigger than q->limits.max_segment_size)?

-- 
Jens Axboe

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