linux-kernel - Re: [PATCH v3 07/11] md/raid5: split bio for chunk_aligned

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Message-ID: <20150508141435.17666f2a@notabene.brown>
Date:	Fri, 8 May 2015 14:14:35 +1000
From:	NeilBrown <neilb@...e.de>
To:	Ming Lin <mlin@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	Jens Axboe <axboe@...nel.dk>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Dongsu Park <dpark@...teo.net>, linux-raid@...r.kernel.org
Subject: Re: [PATCH v3 07/11] md/raid5: split bio for chunk_aligned_read

On Wed,  6 May 2015 23:34:17 -0700 Ming Lin <mlin@...nel.org> wrote:

> If a read request fits entirely in a chunk, it will be passed directly to the
> underlying device (providing it hasn't failed of course).  If it doesn't fit,
> the slightly less efficient path that uses the stripe_cache is used.
> Requests that get to the stripe cache are always completely split up as
> necessary.
> 
> So with RAID5, ripping out the merge_bvec_fn doesn't cause it to stop work,
> but could cause it to take the less efficient path more often.
> 
> All that is needed to manage this is for 'chunk_aligned_read' do some bio
> splitting, much like the RAID0 code does.
> 
> Cc: Neil Brown <neilb@...e.de>
> Cc: linux-raid@...r.kernel.org
> Signed-off-by: Ming Lin <mlin@...nel.org>
> ---
>  drivers/md/raid5.c | 42 +++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 37 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 7f4a717..b18f548 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -4738,7 +4738,7 @@ static void raid5_align_endio(struct bio *bi, int error)
>  	add_bio_to_retry(raid_bi, conf);
>  }
>  
> -static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
> +static int raid5_read_one_chunk(struct mddev *mddev, struct bio *raid_bio)
>  {
>  	struct r5conf *conf = mddev->private;
>  	int dd_idx;
> @@ -4747,7 +4747,7 @@ static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
>  	sector_t end_sector;
>  
>  	if (!in_chunk_boundary(mddev, raid_bio)) {
> -		pr_debug("chunk_aligned_read : non aligned\n");
> +		pr_debug("%s: non aligned\n", __func__);
>  		return 0;
>  	}
>  	/*
> @@ -4822,6 +4822,36 @@ static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
>  	}
>  }
>  
> +static struct bio *chunk_aligned_read(struct mddev *mddev, struct bio *raid_bio)
> +{
> +	struct bio *split;
> +
> +	do {
> +		sector_t sector = raid_bio->bi_iter.bi_sector;
> +		unsigned chunk_sects = mddev->chunk_sectors;
> +		unsigned sectors;
> +
> +		if (likely(is_power_of_2(chunk_sects)))
> +			sectors = chunk_sects - (sector & (chunk_sects-1));
> +		else
> +			sectors = chunk_sects - sector_div(sector, chunk_sects);

RAID5 doesn't currently allow non-power-of-2 chunks.  So this test is
pointless, but not really harmful.  Maybe someday we will.

I'm equally happy for it to stay or go.

Acked-by: NeilBrown <neilb@...e.de>

Thanks,
NeilBrown


> +
> +		if (sectors < bio_sectors(raid_bio)) {
> +			split = bio_split(raid_bio, sectors, GFP_NOIO, fs_bio_set);
> +			bio_chain(split, raid_bio);
> +		} else
> +			split = raid_bio;
> +
> +		if (!raid5_read_one_chunk(mddev, split)) {
> +			if (split != raid_bio)
> +				generic_make_request(raid_bio);
> +			return split;
> +		}
> +	} while (split != raid_bio);
> +
> +	return NULL;
> +}
> +
>  /* __get_priority_stripe - get the next stripe to process
>   *
>   * Full stripe writes are allowed to pass preread active stripes up until
> @@ -5099,9 +5129,11 @@ static void make_request(struct mddev *mddev, struct bio * bi)
>  	 * data on failed drives.
>  	 */
>  	if (rw == READ && mddev->degraded == 0 &&
> -	     mddev->reshape_position == MaxSector &&
> -	     chunk_aligned_read(mddev,bi))
> -		return;
> +	    mddev->reshape_position == MaxSector) {
> +		bi = chunk_aligned_read(mddev, bi);
> +		if (!bi)
> +			return;
> +	}
>  
>  	if (unlikely(bi->bi_rw & REQ_DISCARD)) {
>  		make_discard_request(mddev, bi);


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