linux-kernel - Re: [PATCH v6 05/11] block: remove split code in blkdev_issue_{discard,write

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Date:	Wed, 21 Oct 2015 14:18:12 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Ming Lin <mlin@...nel.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Dongsu Park <dpark@...teo.net>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Ming Lin <ming.l@....samsung.com>,
	linux-nvme@...ts.infradead.org
Subject: Re: [PATCH v6 05/11] block: remove split code in
 blkdev_issue_{discard,write_same}

On Wed, Oct 21 2015 at  1:33pm -0400,
Ming Lin <mlin@...nel.org> wrote:

> On Wed, 2015-10-21 at 12:19 -0400, Mike Snitzer wrote:
> > On Wed, Oct 21 2015 at 12:02pm -0400,
> > Mike Snitzer <snitzer@...hat.com> wrote:
> > 
> > > On Wed, Oct 14 2015 at  9:27am -0400,
> > > Christoph Hellwig <hch@...radead.org> wrote:
> > > 
> > > > On Tue, Oct 13, 2015 at 10:44:11AM -0700, Ming Lin wrote:
> > > > > I just did a quick test with a Samsung 900G NVMe device.
> > > > > mkfs.xfs is OK on 4.3-rc5.
> > > > > 
> > > > > What's your device model? I may find a similar one to try.
> > > > 
> > > > This is a HGST Ultrastar SN100
> > > > 
> > > > Analsys and tentativ fix below:
> > > > 
> > > > blktrace for before the commit:
> > > > 
> > > > 259,0    1        2     0.000002543  2394  G   D 0 + 8388607 [mkfs.xfs]
> > > > 259,0    1        3     0.000008230  2394  I   D 0 + 8388607 [mkfs.xfs]
> > > > 259,0    1        4     0.000031090   207  D   D 0 + 8388607 [kworker/1:1H]
> > > > 259,0    1        5     0.000044869  2394  Q   D 8388607 + 8388607 [mkfs.xfs]
> > > > 259,0    1        6     0.000045992  2394  G   D 8388607 + 8388607 [mkfs.xfs]
> > > > 259,0    1        7     0.000049559  2394  I   D 8388607 + 8388607 [mkfs.xfs]
> > > > 259,0    1        8     0.000061551   207  D   D 8388607 + 8388607 [kworker/1:1H]
> > > > 
> > > > .. and so on.
> > > > 
> > > > blktrace with the commit:
> > > > 
> > > > 259,0    2        1     0.000000000  1228  Q   D 0 + 4194304 [mkfs.xfs]
> > > > 259,0    2        2     0.000002543  1228  G   D 0 + 4194304 [mkfs.xfs]
> > > > 259,0    2        3     0.000010080  1228  I   D 0 + 4194304 [mkfs.xfs]
> > > > 259,0    2        4     0.000082187   267  D   D 0 + 4194304 [kworker/2:1H]
> > > > 259,0    2        5     0.000224869  1228  Q   D 4194304 + 4194304 [mkfs.xfs]
> > > > 259,0    2        6     0.000225835  1228  G   D 4194304 + 4194304 [mkfs.xfs]
> > > > 259,0    2        7     0.000229457  1228  I   D 4194304 + 4194304 [mkfs.xfs]
> > > > 259,0    2        8     0.000238507   267  D   D 4194304 + 4194304 [kworker/2:1H]
> > > > 
> > > > So discards are smaller, but better aligned.  Now if I tweak a single
> > > > line in blk-lib.c to be able to use all of bi_size I get the old I/O
> > > > pattern back and everything works fine again:
> > > > 
> > > > diff --git a/block/blk-lib.c b/block/blk-lib.c
> > > > index bd40292..65b61dc 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;
> > > 
> > > Can we change UINT_MAX >> 9 to rounddown to the first factor of
> > > minimum_io_size?
> > > 
> > > That should work for all devices and for dm-thinp (and dm-cache) in
> > > particular will ensure that all discards that are issued will be a
> > > multiple of the underlying device's blocksize.
> > 
> > Jeff Moyer pointed out having req_sects be a factor of
> > discard_granularity makes more sense.  And I agree.  Same difference in
> > the end (since dm-thinp sets discard_granularity to the thinp
> > blocksize).
> 
> An old version of this patch did use discard_granularity
> https://www.redhat.com/archives/dm-devel/2015-August/msg00000.html
> 
> But you didn't agree.
> https://www.redhat.com/archives/dm-devel/2015-August/msg00001.html
> 
> Maybe we can re-add discard_granularity now?

I disagreed on a more generic level than discard_granularity shaping the
split boundary.

But we are where we are.  If we're going to split (due to 32-bit limits
in bio->bi_iter.bi_size) then we should at least do so in terms of the
support discard_granularity.
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