[PATCH v3 0/2] two generic block layer fixes for 5.9

[PATCH v3 0/2] two generic block layer fixes for 5.9

[Coly Li]

Hi Jens,

These two patches are posted for a while, and have reviewed by several
other developers.

Comparing to previous version, now the discard bio alignment patch can
correctly handles partition offset as Martin suggested. I verify it
with 5.8-rc5 kernel on VMware ESXi 6.5. 

Could you please to take them for Linux v5.9 ?

Thanks in advance.

Coly Li 
---

Coly Li (2):
  block: change REQ_OP_ZONE_RESET and REQ_OP_ZONE_RESET_ALL to be odd
    numbers
  block: improve discard bio alignment in __blkdev_issue_discard()

 block/blk-lib.c           | 31 ++++++++++++++++++++++++++++---
 block/blk.h               | 14 ++++++++++++++
 include/linux/blk_types.h |  8 ++++----
 3 files changed, 46 insertions(+), 7 deletions(-)

-- 
2.26.2

[PATCH v3 1/2] block: change REQ_OP_ZONE_RESET and REQ_OP_ZONE_RESET_ALL to be odd numbers

[Coly Li]

Currently REQ_OP_ZONE_RESET and REQ_OP_ZONE_RESET_ALL are defined as
even numbers 6 and 8, such zone reset bios are treated as READ bios by
bio_data_dir(), which is obviously misleading.

The macro bio_data_dir() is defined in include/linux/bio.h as,
 55 #define bio_data_dir(bio) \
 56         (op_is_write(bio_op(bio)) ? WRITE : READ)

And op_is_write() is defined in include/linux/blk_types.h as,
397 static inline bool op_is_write(unsigned int op)
398 {
399         return (op & 1);
400 }

The convention of op_is_write() is when there is data transfer then the
op code should be odd number, and treat as a write op. bio_data_dir()
treats all bio direction as READ if op_is_write() reports false, and
WRITE if op_is_write() reports true.

Because REQ_OP_ZONE_RESET and REQ_OP_ZONE_RESET_ALL are even numbers,
although they don't transfer data but reporting them as READ bio by
bio_data_dir() is misleading and might be wrong. Because these two
commands will reset the writer pointers of the resetting zones, and all
content after the reset write pointer will be invalid and unaccessible,
obviously they are not READ bios in any means.

This patch changes REQ_OP_ZONE_RESET from 6 to 15, and changes
REQ_OP_ZONE_RESET_ALL from 8 to 17. Now bios with these two op code
can be treated as WRITE by bio_data_dir(). Although they don't transfer
data, now we keep them consistent with REQ_OP_DISCARD and
REQ_OP_WRITE_ZEROES with the ituition that they change on-media content
and should be WRITE request.

Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Jens Axboe <axboe@fb.com>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Keith Busch <kbusch@kernel.org>
Cc: Shaun Tancheff <shaun.tancheff@seagate.com>
---
 include/linux/blk_types.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index ccb895f911b1..447b46a0accf 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -300,12 +300,8 @@ enum req_opf {
 	REQ_OP_DISCARD		= 3,
 	/* securely erase sectors */
 	REQ_OP_SECURE_ERASE	= 5,
-	/* reset a zone write pointer */
-	REQ_OP_ZONE_RESET	= 6,
 	/* write the same sector many times */
 	REQ_OP_WRITE_SAME	= 7,
-	/* reset all the zone present on the device */
-	REQ_OP_ZONE_RESET_ALL	= 8,
 	/* write the zero filled sector many times */
 	REQ_OP_WRITE_ZEROES	= 9,
 	/* Open a zone */
@@ -316,6 +312,10 @@ enum req_opf {
 	REQ_OP_ZONE_FINISH	= 12,
 	/* write data at the current zone write pointer */
 	REQ_OP_ZONE_APPEND	= 13,
+	/* reset a zone write pointer */
+	REQ_OP_ZONE_RESET	= 15,
+	/* reset all the zone present on the device */
+	REQ_OP_ZONE_RESET_ALL	= 17,
 
 	/* SCSI passthrough using struct scsi_request */
 	REQ_OP_SCSI_IN		= 32,
-- 
2.26.2

Re: [PATCH v3 1/2] block: change REQ_OP_ZONE_RESET and REQ_OP_ZONE_RESET_ALL to be odd numbers

[Johannes Thumshirn]

Looks good,
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>

[PATCH v3 2/2] block: improve discard bio alignment in __blkdev_issue_discard()

[Coly Li]

This patch improves discard bio split for address and size alignment in
__blkdev_issue_discard(). The aligned discard bio may help underlying
device controller to perform better discard and internal garbage
collection, and avoid unnecessary internal fragment.

Current discard bio split algorithm in __blkdev_issue_discard() may have
non-discarded fregment on device even the discard bio LBA and size are
both aligned to device's discard granularity size.

Here is the example steps on how to reproduce the above problem.
- On a VMWare ESXi 6.5 update3 installation, create a 51GB virtual disk
  with thin mode and give it to a Linux virtual machine.
- Inside the Linux virtual machine, if the 50GB virtual disk shows up as
  /dev/sdb, fill data into the first 50GB by,
        # dd if=/dev/zero of=/dev/sdb bs=4096 count=13107200
- Discard the 50GB range from offset 0 on /dev/sdb,
        # blkdiscard /dev/sdb -o 0 -l 53687091200
- Observe the underlying mapping status of the device
        # sg_get_lba_status /dev/sdb -m 1048 --lba=0
  descriptor LBA: 0x0000000000000000  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000000000800  blocks: 16773120  deallocated
  descriptor LBA: 0x0000000000fff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000001000000  blocks: 8386560  deallocated
  descriptor LBA: 0x00000000017ff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000001800000  blocks: 8386560  deallocated
  descriptor LBA: 0x0000000001fff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000002000000  blocks: 8386560  deallocated
  descriptor LBA: 0x00000000027ff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000002800000  blocks: 8386560  deallocated
  descriptor LBA: 0x0000000002fff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000003000000  blocks: 8386560  deallocated
  descriptor LBA: 0x00000000037ff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000003800000  blocks: 8386560  deallocated
  descriptor LBA: 0x0000000003fff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000004000000  blocks: 8386560  deallocated
  descriptor LBA: 0x00000000047ff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000004800000  blocks: 8386560  deallocated
  descriptor LBA: 0x0000000004fff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000005000000  blocks: 8386560  deallocated
  descriptor LBA: 0x00000000057ff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000005800000  blocks: 8386560  deallocated
  descriptor LBA: 0x0000000005fff800  blocks: 2048  mapped (or unknown)
  descriptor LBA: 0x0000000006000000  blocks: 6291456  deallocated
  descriptor LBA: 0x0000000006600000  blocks: 0  deallocated

Although the discard bio starts at LBA 0 and has 50<<30 bytes size which
are perfect aligned to the discard granularity, from the above list
these are many 1MB (2048 sectors) internal fragments exist unexpectedly.

The problem is in __blkdev_issue_discard(), an improper algorithm causes
an improper bio size which is not aligned.

 25 int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 26                 sector_t nr_sects, gfp_t gfp_mask, int flags,
 27                 struct bio **biop)
 28 {
 29         struct request_queue *q = bdev_get_queue(bdev);
   [snipped]
 56
 57         while (nr_sects) {
 58                 sector_t req_sects = min_t(sector_t, nr_sects,
 59                                 bio_allowed_max_sectors(q));
 60
 61                 WARN_ON_ONCE((req_sects << 9) > UINT_MAX);
 62
 63                 bio = blk_next_bio(bio, 0, gfp_mask);
 64                 bio->bi_iter.bi_sector = sector;
 65                 bio_set_dev(bio, bdev);
 66                 bio_set_op_attrs(bio, op, 0);
 67
 68                 bio->bi_iter.bi_size = req_sects << 9;
 69                 sector += req_sects;
 70                 nr_sects -= req_sects;
   [snipped]
 79         }
 80
 81         *biop = bio;
 82         return 0;
 83 }
 84 EXPORT_SYMBOL(__blkdev_issue_discard);

At line 58-59, to discard a 50GB range, req_sects is set as return value
of bio_allowed_max_sectors(q), which is 8388607 sectors. In the above
case, the discard granularity is 2048 sectors, although the start LBA
and discard length are aligned to discard granularity, req_sects never
has chance to be aligned to discard granularity. This is why there are
some still-mapped 2048 sectors fragment in every 4 or 8 GB range.

If req_sects at line 58 is set to a value aligned to discard_granularity
and close to UNIT_MAX, then all consequent split bios inside device
driver are (almostly) aligned to discard_granularity of the device
queue. The 2048 sectors still-mapped fragment will disappear.

This patch introduces bio_aligned_discard_max_sectors() to return the
the value which is aligned to q->limits.discard_granularity and closest
to UINT_MAX. Then this patch replaces bio_allowed_max_sectors() with
this new routine to decide a more proper split bio length.

But we still need to handle the situation when discard start LBA is not
aligned to q->limits.discard_granularity, otherwise even the length is
aligned, current code may still leave 2048 fragment around every 4GB
range. Therefore, to calculate req_sects, firstly the start LBA of
discard range is checked (including partition offset), if it is not
aligned to discard granularity, the first split location should make
sure following bio has bi_sector aligned to discard granularity. Then
there won't be still-mapped fragment in the middle of the discard range.

The above is how this patch improves discard bio alignment in
__blkdev_issue_discard(). Now with this patch, after discard with same
command line mentiond previously, sg_get_lba_status returns,
descriptor LBA: 0x0000000000000000  blocks: 106954752  deallocated
descriptor LBA: 0x0000000006600000  blocks: 0  deallocated

We an see there is no 2048 sectors segment anymore, everything is clean.

Reported-and-tested-by: Acshai Manoj <acshai.manoj@microfocus.com>
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Enzo Matsumiya <ematsumiya@suse.com>
Cc: Jens Axboe <axboe@kernel.dk>
---
Changelog:
v3: handle partition offset suggested by Martin.
v2: fix the overflow pointed by Ming Lei.
v1: initial version.

 block/blk-lib.c | 31 ++++++++++++++++++++++++++++---
 block/blk.h     | 14 ++++++++++++++
 2 files changed, 42 insertions(+), 3 deletions(-)

diff --git a/block/blk-lib.c b/block/blk-lib.c
index 5f2c429d4378..019e09bb9c0e 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -29,7 +29,7 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 	struct request_queue *q = bdev_get_queue(bdev);
 	struct bio *bio = *biop;
 	unsigned int op;
-	sector_t bs_mask;
+	sector_t bs_mask, part_offset = 0;
 
 	if (!q)
 		return -ENXIO;
@@ -54,9 +54,34 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 	if (!nr_sects)
 		return -EINVAL;
 
+	/* In case the discard request is in a partition */
+	if (bdev->bd_partno)
+		part_offset = bdev->bd_part->start_sect;
+
 	while (nr_sects) {
-		sector_t req_sects = min_t(sector_t, nr_sects,
-				bio_allowed_max_sectors(q));
+		sector_t granularity_aligned_lba, req_sects;
+		sector_t sector_mapped = sector + part_offset;
+
+		granularity_aligned_lba = round_up(sector_mapped,
+				q->limits.discard_granularity >> SECTOR_SHIFT);
+
+		/*
+		 * Check whether the discard bio starts at a discard_granularity
+		 * aligned LBA,
+		 * - If no: set (granularity_aligned_lba - sector_mapped) to
+		 *   bi_size of the first split bio, then the second bio will
+		 *   start at a discard_granularity aligned LBA on the device.
+		 * - If yes: use bio_aligned_discard_max_sectors() as the max
+		 *   possible bi_size of the first split bio. Then when this bio
+		 *   is split in device drive, the split ones are very probably
+		 *   to be aligned to discard_granularity of the device's queue.
+		 */
+		if (granularity_aligned_lba == sector_mapped)
+			req_sects = min_t(sector_t, nr_sects,
+					  bio_aligned_discard_max_sectors(q));
+		else
+			req_sects = min_t(sector_t, nr_sects,
+					  granularity_aligned_lba - sector_mapped);
 
 		WARN_ON_ONCE((req_sects << 9) > UINT_MAX);
 
diff --git a/block/blk.h b/block/blk.h
index b5d1f0fc6547..a80738581f84 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -281,6 +281,20 @@ static inline unsigned int bio_allowed_max_sectors(struct request_queue *q)
 	return round_down(UINT_MAX, queue_logical_block_size(q)) >> 9;
 }
 
+/*
+ * The max bio size which is aligned to q->limits.discard_granularity. This
+ * is a hint to split large discard bio in generic block layer, then if device
+ * driver needs to split the discard bio into smaller ones, their bi_size can
+ * be very probably and easily aligned to discard_granularity of the device's
+ * queue.
+ */
+static inline unsigned int bio_aligned_discard_max_sectors(
+					struct request_queue *q)
+{
+	return round_down(UINT_MAX, q->limits.discard_granularity) >>
+			SECTOR_SHIFT;
+}
+
 /*
  * Internal io_context interface
  */
-- 
2.26.2

Re: [PATCH v3 0/2] two generic block layer fixes for 5.9

[Jens Axboe]

On 7/16/20 8:42 PM, Coly Li wrote:
> Hi Jens,
> 
> These two patches are posted for a while, and have reviewed by several
> other developers.
> 
> Comparing to previous version, now the discard bio alignment patch can
> correctly handles partition offset as Martin suggested. I verify it
> with 5.8-rc5 kernel on VMware ESXi 6.5. 
> 
> Could you please to take them for Linux v5.9 ?

Applied, thanks.

-- 
Jens Axboe