On 2020/5/30 06:55, Ming Lei wrote:
> 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@xxxxxxxxxxxxxx>
>> Signed-off-by: Coly Li <colyli@xxxxxxx>
>> Cc: Bart Van Assche <bvanassche@xxxxxxx>
>> Cc: Christoph Hellwig <hch@xxxxxx>
>> Cc: Enzo Matsumiya <ematsumiya@xxxxxxxx>
>> Cc: Hannes Reinecke <hare@xxxxxxxx>
>> Cc: Jens Axboe <axboe@xxxxxxxxx>
>> Cc: Ming Lei <ming.lei@xxxxxxxxxx>
>> Cc: Xiao Ni <xni@xxxxxxxxxx>
>> ---
>> 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.
It seems no value in these two min_t() can be zero.
Could you please give me more hint ?
>>
>> 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.
It seems like I should use roundup() indeed. Thanks for the hint, let me
improve in v3 patch.
Coly Li
