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

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Date:	Wed, 14 Oct 2015 16:38:50 +0000 (UTC)
From:	Keith Busch <keith.busch@...el.com>
To:	Christoph Hellwig <hch@...radead.org>
cc:	Ming Lin <mlin@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Ming Lin <ming.l@....samsung.com>,
	Mike Snitzer <snitzer@...hat.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-nvme@...ts.infradead.org, Dongsu Park <dpark@...teo.net>,
	Kent Overstreet <kent.overstreet@...il.com>
Subject: Re: [PATCH v6 05/11] block: remove split code in
 blkdev_issue_{discard,write_same}B

On Wed, 14 Oct 2015, Christoph Hellwig wrote:
> 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:

I see why the proposal is an improvement, but I don't understand why the
current situation results in a hang. Are we missing some kind of error
recovery in the driver?
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