Re: [PATCH]bcache : limit the bio max sectors to make request bug in raid0

[Jack Wang <jinpu.wang-EIkl63zCoXaH+58JC4qpiA@public.gmane.org>]

On 2013年04月22日 23:46, Kent Overstreet wrote:
> On Wed, Apr 17, 2013 at 09:36:11AM +0200, Jack Wang wrote:
>> We are using your bcache-testing branch.
>>
>> From 20ad8cfb8047df2d09a5a960610f02c555a31a4f Mon Sep 17 00:00:00 2001
>> From: Jack Wang <jinpu.wang-EIkl63zCoXaH+58JC4qpiA@public.gmane.org>
>> Date: Tue, 16 Apr 2013 14:59:04 +0200
>> Subject: [PATCH] limit the max sectors in bcache to fix the make request bug
>>  in raid10
>> During test bcache with raid1+0, we saw a lot of complain as below:
>> [ 2766.555172] md/raid0:md400: make_request bug: can't convert block
>> across chunks or bigger than 512k 953328 144
>>
>> when the using dd or fio with bigger blocksize like 512k, limited the
>> bio_max_sectors resolve this issue.
> 
> The fix is wrong - it'll have the effect of limiting _every_ bio bcache
> emits to a single page, which will be bad for performance.
> 
> I think I just found the problem though - looking at the raid0
> merge_bvec_fn, when it's stacked on top of devices with their own
> merge_bvec_fns, it reuses the bvm was passed to it and modifies the
> bi_bdev and bi_sector. Ew.
> 
> Can you try this patch and see if it fixes it?
Sure, thanks for looking into it, will test it today.
> 
> commit a09ded8edf9ed4009930713e101249084cbcea5c
> Author: Kent Overstreet <koverstreet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> Date:   Mon Apr 22 14:44:24 2013 -0700
> 
>     bcache: Fix merge_bvec_fn usage for when it modifies the bvm
>     
>     Stacked md devices reuse the bvm for the subordinate device, causing
>     problems...
>     
>     Reported-by: Michael Balser <michael.balser-EIkl63zCoXaH+58JC4qpiA@public.gmane.org>
>     Signed-off-by: Kent Overstreet <koverstreet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> 
> diff --git a/drivers/md/bcache/io.c b/drivers/md/bcache/io.c
> index 5304eaa..48efd4d 100644
> --- a/drivers/md/bcache/io.c
> +++ b/drivers/md/bcache/io.c
> @@ -163,13 +163,6 @@ static unsigned bch_bio_max_sectors(struct bio *bio)
>  	struct bio_vec *bv, *end = bio_iovec(bio) +
>  		min_t(int, bio_segments(bio), max_segments);
>  
> -	struct bvec_merge_data bvm = {
> -		.bi_bdev	= bio->bi_bdev,
> -		.bi_sector	= bio->bi_sector,
> -		.bi_size	= 0,
> -		.bi_rw		= bio->bi_rw,
> -	};
> -
>  	if (bio->bi_rw & REQ_DISCARD)
>  		return min(ret, q->limits.max_discard_sectors);
>  
> @@ -178,12 +171,18 @@ static unsigned bch_bio_max_sectors(struct bio *bio)
>  		ret = 0;
>  
>  		for (bv = bio_iovec(bio); bv < end; bv++) {
> +			struct bvec_merge_data bvm = {
> +				.bi_bdev	= bio->bi_bdev,
> +				.bi_sector	= bio->bi_sector,
> +				.bi_size	= ret << 9,
> +				.bi_rw		= bio->bi_rw,
> +			};
> +
>  			if (q->merge_bvec_fn &&
>  			    q->merge_bvec_fn(q, &bvm, bv) < (int) bv->bv_len)
>  				break;
>  
> -			ret		+= bv->bv_len >> 9;
> -			bvm.bi_size	+= bv->bv_len;
> +			ret += bv->bv_len >> 9;
>  		}
>  	}
>  
>