Re: [PATCH v6 05/11] block: remove split code in blkdev_issue_{discard,write_same}

From: Ming Lin
Date: Wed Oct 21 2015 - 16:13:25 EST

On Wed, 2015-10-21 at 14:18 -0400, Mike Snitzer wrote:
> On Wed, Oct 21 2015 at 1:33pm -0400,
> Ming Lin <mlin@xxxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > >
> > > > On Wed, Oct 14 2015 at 9:27am -0400,
> > > > Christoph Hellwig <hch@xxxxxxxxxxxxx> 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.

How about below?
It actually reverts commit b49a0871 and adds patch at
https://www.redhat.com/archives/dm-devel/2015-August/msg00000.html

Christoph, could you help to try it?

commit 122bf0a43cb1611ed62aaf945f25b649c27a71ed
Author: Ming Lin <mlin@xxxxxxxxxx>
Date: Wed Oct 21 11:24:48 2015 -0700

block: check discard_granularity and alignment

Signed-off-by: Ming Lin <ming.l@xxxxxxxxxxxxxxx>
---
block/blk-lib.c | 31 ++++++++++++++++++++++---------
1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/block/blk-lib.c b/block/blk-lib.c
index bd40292..9ebf653 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -26,13 +26,6 @@ static void bio_batch_end_io(struct bio *bio)
bio_put(bio);
}

-/*
- * Ensure that max discard sectors doesn't overflow bi_size and hopefully
- * it is of the proper granularity as long as the granularity is a power
- * of two.
- */
-#define MAX_BIO_SECTORS ((1U << 31) >> 9)
-
/**
* blkdev_issue_discard - queue a discard
* @bdev: blockdev to issue discard for
@@ -50,6 +43,8 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
DECLARE_COMPLETION_ONSTACK(wait);
struct request_queue *q = bdev_get_queue(bdev);
int type = REQ_WRITE | REQ_DISCARD;
+ unsigned int granularity;
+ int alignment;
struct bio_batch bb;
struct bio *bio;
int ret = 0;
@@ -61,6 +56,10 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
if (!blk_queue_discard(q))
return -EOPNOTSUPP;

+ /* Zero-sector (unknown) and one-sector granularities are the same. */
+ granularity = max(q->limits.discard_granularity >> 9, 1U);
+ alignment = (bdev_discard_alignment(bdev) >> 9) % granularity;
+
if (flags & BLKDEV_DISCARD_SECURE) {
if (!blk_queue_secdiscard(q))
return -EOPNOTSUPP;
@@ -74,7 +73,7 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
blk_start_plug(&plug);
while (nr_sects) {
unsigned int req_sects;
- sector_t end_sect;
+ sector_t end_sect, tmp;

bio = bio_alloc(gfp_mask, 1);
if (!bio) {
@@ -82,8 +81,22 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
break;
}

- req_sects = min_t(sector_t, nr_sects, MAX_BIO_SECTORS);
+ /* Make sure bi_size doesn't overflow */
+ req_sects = min_t(sector_t, nr_sects, UINT_MAX >> 9);
+
+ /*
+ * If splitting a request, and the next starting sector would be
+ * misaligned, stop the discard at the previous aligned sector.
+ */
end_sect = sector + req_sects;
+ tmp = end_sect;
+ if (req_sects < nr_sects &&
+ sector_div(tmp, granularity) != alignment) {
+ end_sect = end_sect - alignment;
+ sector_div(end_sect, granularity);
+ end_sect = end_sect * granularity + alignment;
+ req_sects = end_sect - sector;
+ }

bio->bi_iter.bi_sector = sector;
bio->bi_end_io = bio_batch_end_io;


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