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

[Hannes Reinecke <hare@suse.de>]

On 5/30/20 3:52 PM, 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_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, 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-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, the improved version with inspire from review comments by Bart,
>      Ming and Xiao.
> v1, the initial version.
> 
>   block/blk-lib.c | 25 +++++++++++++++++++++++--
>   block/blk.h     | 14 ++++++++++++++
>   2 files changed, 37 insertions(+), 2 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
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