On Sat, May 30, 2020 at 08:53:56PM +0800, Ming Lei wrote:
> On Sat, May 30, 2020 at 06:07:07PM +0800, Coly Li wrote:
> > 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 ?
>
> Looks I misunderstood it, so it is fine in this way.
>
> >
> >
> >
> > >>
> > >> 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.
>
> Actually, bio_aligned_discard_max_sectors() needn't to be <=
> q->limits.max_discard_sectors because we will split this discard
> request.
>
> Thinking of the issue further, the above stuff should have been done
> in blk_bio_discard_split() instead of __blkdev_issue_discard() in which
> we should simply create/submit one non-overflow bio, and shouldn't care
> the granularity aligned stuff. blk_bio_discard_split() is supposed to
> consider all kinds of queue limit and decide how to split.
oops, I know the story now, that is we only have 32bit .bi_size, so
split code can't make prefect discard bio.
Then your patch is fine after overflow is fixed in bio_aligned_discard_max_sectors,
given detailed comment is provided.
Thanks,
Ming
