[dm-devel] [PATCH v5 01/11] block: make generic_make_request handle arbitrarily sized bios

[Mike Snitzer]

On Tue, Aug 18 2015 at  1:32pm -0400,
Ming Lin <mlin at kernel.org> wrote:

> On Tue, 2015-08-18 at 10:45 -0400, Mike Snitzer wrote:
> > On Tue, Aug 18 2015 at  3:04am -0400,
> > Ming Lin <mlin at kernel.org> wrote:
> > 
> > > On Mon, Aug 17, 2015 at 10:09 PM, Ming Lin <mlin at kernel.org> wrote:
> > > > On Mon, Aug 10, 2015 at 8:02 AM, Mike Snitzer <snitzer at redhat.com> wrote:
> > > >> p.s. I'll be working with Joe Thornber on optimizing DM (particularly
> > > >> dm-thinp and dm-cache) once this patchset is included upstream.  You'll
> > > >> see I've already added a couple WIP dm-thinp patches ontop.
> > > >
> > > > Hi Mike,
> > > >
> > > > Just to avoid duplicated work.
> > > > Are you going to work on the dm-thinp/dm-cache discard rewritten?
> > > 
> > > Seems dm-stripe discard also needs rewrite.
> > 
> > Can you elaborate on what you feel needs re-writing in these targets?
> 
> dm-stripe also require discard size to be a multiple of chunk size.
> See output of below debug patch for 4G discard.
> 
> root at bee:~# blkdiscard -o 0 -l 4294967296 /dev/striped_vol_group/striped_logical_volume
> 
> root at bee:~# dmesg |grep DEBUG
> [   13.110224] DEBUG: discard ignored: stripe chunk size 128K bytes, bio size 512 bytes
> [   13.113723] DEBUG: discard ignored: stripe chunk size 128K bytes, bio size 512 bytes
> [   13.117098] DEBUG: discard ignored: stripe chunk size 128K bytes, bio size 512 bytes
> [   13.120424] DEBUG: discard ignored: stripe chunk size 128K bytes, bio size 512 bytes
> [   13.123800] DEBUG: discard ignored: stripe chunk size 128K bytes, bio size 512 bytes
> [   13.127027] DEBUG: discard ignored: stripe chunk size 128K bytes, bio size 512 bytes
> [   13.130161] DEBUG: discard ignored: stripe chunk size 128K bytes, bio size 512 bytes
> 
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index bd40292..1cab2ba 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -82,7 +82,7 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>  			break;
>  		}
>  
> -		req_sects = min_t(sector_t, nr_sects, MAX_BIO_SECTORS);
> +		req_sects = min_t(sector_t, nr_sects, UINT_MAX>>9);
>  		end_sect = sector + req_sects;
>  
>  		bio->bi_iter.bi_sector = sector;
> diff --git a/drivers/md/dm-stripe.c b/drivers/md/dm-stripe.c
> index 484029d..a288bc2 100644
> --- a/drivers/md/dm-stripe.c
> +++ b/drivers/md/dm-stripe.c
> @@ -273,6 +273,8 @@ static int stripe_map_range(struct stripe_c *sc, struct bio *bio,
>  		return DM_MAPIO_REMAPPED;
>  	} else {
>  		/* The range doesn't map to the target stripe */
> +		printk("DEBUG: discard ignored: stripe chunk size %dK bytes, bio size %d bytes\n",
> +			sc->chunk_size>>1, bio->bi_iter.bi_size);
>  		bio_endio(bio);
>  		return DM_MAPIO_SUBMITTED;
>  	}

This is expected.  If a discard is only 512 bytes and the chunk size is
128K then every discard will only ever hit one stripe.

So each discard will have N - 1 "discard ignored" messages (when N is #
of stripes in the dm-stripe device).  So in your test device I'd assume
you have 8 stripes.

Basically your debugging looks like it is _very_ prone to false
positives here.  The dm-stripe code is working as expected.