Re: [PATCH v2] block: improve discard bio alignment in __blkdev_issue_discard()

[Ming Lei <ming.lei@redhat.com>]

On Sat, May 30, 2020 at 12:34:18AM +0800, Coly Li wrote:
> 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_sets 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, seq_sets never
> has chance to be aligned to discard granularity. This is why there are
> some still-mapped 2048 sectors segment in every 4 or 8 GB range.
> 
> Because queue's max_discard_sectors is aligned to discard granularity,
> if req_sects at line 58 is set to a value closest to UINT_MAX and
> aligned to q->limits.max_discard_sectors, then all consequent split bios
> inside device driver are (almostly) aligned to discard_granularity of
> the device queue.
> 
> This patch introduces bio_aligned_discard_max_sectors() to return the
> closet to UINT_MAX and aligned to q->limits.discard_granularity value,
> and replace bio_allowed_max_sectors() with this new inline routine to
> decide the 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 segment around every 4BG
> range. Thereforeto calculate req_sects, firstly the start LBA of discard
> request command is checked, 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 segment
> 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-by: Acshai Manoj <acshai.manoj@microfocus.com>
> Signed-off-by: Coly Li <colyli@suse.de>
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Enzo Matsumiya <ematsumiya@suse.com>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Xiao Ni <xni@redhat.com>
> ---
> Changelog:
> v2: replace 9 with SECTOR_SHIFT as suggested by Bart Van Assche.
> v1: initial version.
> 
>  block/blk-lib.c | 25 +++++++++++++++++++++++--
>  block/blk.h     | 15 +++++++++++++++
>  2 files changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index 5f2c429d4378..2fc0e3cc1ed8 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -55,8 +55,29 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>  		return -EINVAL;
>  
>  	while (nr_sects) {
> -		sector_t req_sects = min_t(sector_t, nr_sects,
> -				bio_allowed_max_sectors(q));
> +		sector_t granularity_aligned_lba;
> +		sector_t req_sects;
> +
> +		granularity_aligned_lba =
> +			round_up(sector, q->limits.discard_granularity);
> +
> +		/*
> +		 * Check whether the discard bio starts at a discard_granularity
> +		 * aligned LBA,
> +		 * - If no: set (granularity_aligned_lba - sector) to bi_size of
> +		 *   the first split bio, then the second bio will start at a
> +		 *   discard_granularity aligned LBA.
> +		 * - If yes: use bio_aligned_discard_max_sectors() as the max
> +		 *   possible bi_size of th first split bio. Then when this bio
> +		 *   is split in device drive, the split ones are always easier
> +		 *   to be aligned to max_discard_sectors of the device's queue.
> +		 */
> +		if (granularity_aligned_lba == sector)
> +			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);

min_non_zero() may be cleaner.

>  
>  		WARN_ON_ONCE((req_sects << 9) > UINT_MAX);
>  
> diff --git a/block/blk.h b/block/blk.h
> index 0a94ec68af32..dc5369e7e1fb 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -292,6 +292,21 @@ 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.max_discard_sectors. 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 ligned to max_discard_sectors 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.max_discard_sectors << SECTOR_SHIFT))
> +			>> SECTOR_SHIFT;
> +}

The above may not be correct, what if q->limits.max_discard_sectors is
less enough? raid10 may use default 512k max discard bytes. Then
bio_aligned_discard_max_sectors() can return bigger value than
q->limits.max_discard_sectors, and breaks this discard limit.

thanks,
Ming