Re: [PATCH block v2 2/3] block: Add support for REQ_NOZERO flag

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 19.01.2020 04:50, Bob Liu wrote:
> On 1/16/20 8:36 PM, Kirill Tkhai wrote:
>> This adds support for REQ_NOZERO extension of REQ_OP_WRITE_ZEROES
>> operation, which encourages a block device driver to just allocate
>> blocks (or mark them allocated) instead of actual blocks zeroing.
>> REQ_NOZERO is aimed to be used for network filesystems providing
>> a block device interface. Also, block devices, which map a file
>> on other filesystem (like loop), may use this for less fragmentation
>> and batching fallocate() requests. Hypervisors like QEMU may
>> introduce optimizations of clusters allocations based on this.
>>
>> BLKDEV_ZERO_ALLOCATE is a new corresponding flag for
>> blkdev_issue_zeroout().
>>> CC: Martin K. Petersen <martin.petersen@oracle.com>
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> ---
>>  block/blk-core.c          |    6 +++---
>>  block/blk-lib.c           |   17 ++++++++++-------
>>  block/blk-merge.c         |    9 ++++++---
>>  block/blk-settings.c      |   17 +++++++++++++++++
>>  fs/block_dev.c            |    4 ++++
>>  include/linux/blk_types.h |    5 ++++-
>>  include/linux/blkdev.h    |   31 ++++++++++++++++++++++++-------
>>  7 files changed, 68 insertions(+), 21 deletions(-)
>>
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 50a5de025d5e..2edcd55624f1 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -978,7 +978,7 @@ generic_make_request_checks(struct bio *bio)
>>  			goto not_supported;
>>  		break;
>>  	case REQ_OP_WRITE_ZEROES:
>> -		if (!q->limits.max_write_zeroes_sectors)
>> +		if (!blk_queue_get_max_write_zeroes_sectors(q, bio->bi_opf))
>>  			goto not_supported;
>>  		break;
>>  	default:
>> @@ -1250,10 +1250,10 @@ EXPORT_SYMBOL(submit_bio);
>>  static int blk_cloned_rq_check_limits(struct request_queue *q,
>>  				      struct request *rq)
>>  {
>> -	if (blk_rq_sectors(rq) > blk_queue_get_max_sectors(q, req_op(rq))) {
>> +	if (blk_rq_sectors(rq) > blk_queue_get_max_sectors(q, rq->cmd_flags)) {
>>  		printk(KERN_ERR "%s: over max size limit. (%u > %u)\n",
>>  			__func__, blk_rq_sectors(rq),
>> -			blk_queue_get_max_sectors(q, req_op(rq)));
>> +			blk_queue_get_max_sectors(q, rq->cmd_flags));
>>  		return -EIO;
>>  	}
>>  
>> diff --git a/block/blk-lib.c b/block/blk-lib.c
>> index 3e38c93cfc53..3e80279eb029 100644
>> --- a/block/blk-lib.c
>> +++ b/block/blk-lib.c
>> @@ -214,7 +214,7 @@ static int __blkdev_issue_write_zeroes(struct block_device *bdev,
>>  		struct bio **biop, unsigned flags)
>>  {
>>  	struct bio *bio = *biop;
>> -	unsigned int max_write_zeroes_sectors;
>> +	unsigned int max_write_zeroes_sectors, req_flags = 0;
>>  	struct request_queue *q = bdev_get_queue(bdev);
>>  
>>  	if (!q)
>> @@ -224,18 +224,21 @@ static int __blkdev_issue_write_zeroes(struct block_device *bdev,
>>  		return -EPERM;
>>  
>>  	/* Ensure that max_write_zeroes_sectors doesn't overflow bi_size */
>> -	max_write_zeroes_sectors = bdev_write_zeroes_sectors(bdev, 0);
>> +	max_write_zeroes_sectors = bdev_write_zeroes_sectors(bdev, flags);
>>  
>>  	if (max_write_zeroes_sectors == 0)
>>  		return -EOPNOTSUPP;
>>  
>> +	if (flags & BLKDEV_ZERO_NOUNMAP)
>> +		req_flags |= REQ_NOUNMAP;
>> +	if (flags & BLKDEV_ZERO_ALLOCATE)
>> +		req_flags |= REQ_NOZERO|REQ_NOUNMAP;
>> +
>>  	while (nr_sects) {
>>  		bio = blk_next_bio(bio, 0, gfp_mask);
>>  		bio->bi_iter.bi_sector = sector;
>>  		bio_set_dev(bio, bdev);
>> -		bio->bi_opf = REQ_OP_WRITE_ZEROES;
>> -		if (flags & BLKDEV_ZERO_NOUNMAP)
>> -			bio->bi_opf |= REQ_NOUNMAP;
>> +		bio->bi_opf = REQ_OP_WRITE_ZEROES | req_flags;
>>  
>>  		if (nr_sects > max_write_zeroes_sectors) {
>>  			bio->bi_iter.bi_size = max_write_zeroes_sectors << 9;
>> @@ -362,7 +365,7 @@ int blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
>>  	sector_t bs_mask;
>>  	struct bio *bio;
>>  	struct blk_plug plug;
>> -	bool try_write_zeroes = !!bdev_write_zeroes_sectors(bdev, 0);
>> +	bool try_write_zeroes = !!bdev_write_zeroes_sectors(bdev, flags);
>>  
>>  	bs_mask = (bdev_logical_block_size(bdev) >> 9) - 1;
>>  	if ((sector | nr_sects) & bs_mask)
>> @@ -391,7 +394,7 @@ int blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
>>  			try_write_zeroes = false;
>>  			goto retry;
>>  		}
>> -		if (!bdev_write_zeroes_sectors(bdev, 0)) {
>> +		if (!bdev_write_zeroes_sectors(bdev, flags)) {
>>  			/*
>>  			 * Zeroing offload support was indicated, but the
>>  			 * device reported ILLEGAL REQUEST (for some devices
>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>> index 347782a24a35..e3ce4b87bbaa 100644
>> --- a/block/blk-merge.c
>> +++ b/block/blk-merge.c
>> @@ -105,15 +105,18 @@ static struct bio *blk_bio_discard_split(struct request_queue *q,
>>  static struct bio *blk_bio_write_zeroes_split(struct request_queue *q,
>>  		struct bio *bio, struct bio_set *bs, unsigned *nsegs)
>>  {
>> +	unsigned int max_sectors;
>> +
>> +	max_sectors = blk_queue_get_max_write_zeroes_sectors(q, bio->bi_opf);
>>  	*nsegs = 0;
>>  
>> -	if (!q->limits.max_write_zeroes_sectors)
>> +	if (!max_sectors)
>>  		return NULL;
>>  
>> -	if (bio_sectors(bio) <= q->limits.max_write_zeroes_sectors)
>> +	if (bio_sectors(bio) <= max_sectors)
>>  		return NULL;
>>  
>> -	return bio_split(bio, q->limits.max_write_zeroes_sectors, GFP_NOIO, bs);
>> +	return bio_split(bio, max_sectors, GFP_NOIO, bs);
>>  }
>>  
>>  static struct bio *blk_bio_write_same_split(struct request_queue *q,
>> diff --git a/block/blk-settings.c b/block/blk-settings.c
>> index 5f6dcc7a47bd..f682374c5106 100644
>> --- a/block/blk-settings.c
>> +++ b/block/blk-settings.c
>> @@ -48,6 +48,7 @@ void blk_set_default_limits(struct queue_limits *lim)
>>  	lim->chunk_sectors = 0;
>>  	lim->max_write_same_sectors = 0;
>>  	lim->max_write_zeroes_sectors = 0;
>> +	lim->max_allocate_sectors = 0;
>>  	lim->max_discard_sectors = 0;
>>  	lim->max_hw_discard_sectors = 0;
>>  	lim->discard_granularity = 0;
>> @@ -83,6 +84,7 @@ void blk_set_stacking_limits(struct queue_limits *lim)
>>  	lim->max_dev_sectors = UINT_MAX;
>>  	lim->max_write_same_sectors = UINT_MAX;
>>  	lim->max_write_zeroes_sectors = UINT_MAX;
>> +	lim->max_allocate_sectors = UINT_MAX;
>>  }
>>  EXPORT_SYMBOL(blk_set_stacking_limits);
>>  
>> @@ -257,6 +259,19 @@ void blk_queue_max_write_zeroes_sectors(struct request_queue *q,
>>  }
>>  EXPORT_SYMBOL(blk_queue_max_write_zeroes_sectors);
>>  
>> +/**
>> + * blk_queue_max_allocate_sectors - set max sectors for a single
>> + *                                  allocate request
>> + * @q:  the request queue for the device
>> + * @max_allocate_sectors: maximum number of sectors to write per command
>> + **/
>> +void blk_queue_max_allocate_sectors(struct request_queue *q,
>> +		unsigned int max_allocate_sectors)
>> +{
>> +	q->limits.max_allocate_sectors = max_allocate_sectors;
>> +}
>> +EXPORT_SYMBOL(blk_queue_max_allocate_sectors);
>> +
> 
> I'd suggest split this to a separated patch.

Yeah, this function is used in [3/3] only, so in may go after this patch [2/3] as a separate patch.

>>  /**
>>   * blk_queue_max_segments - set max hw segments for a request for this queue
>>   * @q:  the request queue for the device
>> @@ -506,6 +521,8 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
>>  					b->max_write_same_sectors);
>>  	t->max_write_zeroes_sectors = min(t->max_write_zeroes_sectors,
>>  					b->max_write_zeroes_sectors);
>> +	t->max_allocate_sectors = min(t->max_allocate_sectors,
>> +					b->max_allocate_sectors);
>>  	t->bounce_pfn = min_not_zero(t->bounce_pfn, b->bounce_pfn);
>>  
>>  	t->seg_boundary_mask = min_not_zero(t->seg_boundary_mask,
>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>> index 69bf2fb6f7cd..1ffef894b3bd 100644
>> --- a/fs/block_dev.c
>> +++ b/fs/block_dev.c
>> @@ -2122,6 +2122,10 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
>>  		error = blkdev_issue_zeroout(bdev, start >> 9, len >> 9,
>>  					     GFP_KERNEL, BLKDEV_ZERO_NOFALLBACK);
>>  		break;
>> +	case FALLOC_FL_KEEP_SIZE:
>> +		error = blkdev_issue_zeroout(bdev, start >> 9, len >> 9,
>> +			GFP_KERNEL, BLKDEV_ZERO_ALLOCATE | BLKDEV_ZERO_NOFALLBACK);
>> +		break;
>>  	case FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE | FALLOC_FL_NO_HIDE_STALE:
>>  		error = blkdev_issue_discard(bdev, start >> 9, len >> 9,
>>  					     GFP_KERNEL, 0);
>> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
>> index 70254ae11769..9ed166860099 100644
>> --- a/include/linux/blk_types.h
>> +++ b/include/linux/blk_types.h
>> @@ -335,7 +335,9 @@ enum req_flag_bits {
>>  
>>  	/* command specific flags for REQ_OP_WRITE_ZEROES: */
>>  	__REQ_NOUNMAP,		/* do not free blocks when zeroing */
>> -
>> +	__REQ_NOZERO,		/* only notify about allocated blocks,
>> +				 * and do not actual zero them
>> +				 */
>>  	__REQ_HIPRI,
>>  
>>  	/* for driver use */
>> @@ -362,6 +364,7 @@ enum req_flag_bits {
>>  #define REQ_CGROUP_PUNT		(1ULL << __REQ_CGROUP_PUNT)
>>  
>>  #define REQ_NOUNMAP		(1ULL << __REQ_NOUNMAP)
>> +#define REQ_NOZERO		(1ULL << __REQ_NOZERO)
>>  #define REQ_HIPRI		(1ULL << __REQ_HIPRI)
>>  
>>  #define REQ_DRV			(1ULL << __REQ_DRV)
>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>> index 4cd69552df9a..f4ec5db64432 100644
>> --- a/include/linux/blkdev.h
>> +++ b/include/linux/blkdev.h
>> @@ -336,6 +336,7 @@ struct queue_limits {
>>  	unsigned int		max_hw_discard_sectors;
>>  	unsigned int		max_write_same_sectors;
>>  	unsigned int		max_write_zeroes_sectors;
>> +	unsigned int		max_allocate_sectors;
>>  	unsigned int		discard_granularity;
>>  	unsigned int		discard_alignment;
>>  
>> @@ -988,9 +989,19 @@ static inline struct bio_vec req_bvec(struct request *rq)
>>  	return mp_bvec_iter_bvec(rq->bio->bi_io_vec, rq->bio->bi_iter);
>>  }
>>  
>> +static inline unsigned int blk_queue_get_max_write_zeroes_sectors(
>> +		struct request_queue *q, unsigned int op_flags)
>> +{
>> +	if (op_flags & REQ_NOZERO)
>> +		return q->limits.max_allocate_sectors;
>> +	return q->limits.max_write_zeroes_sectors;
>> +}
>> +
> 
> And this one.

It looks it won't be good, since this will require to declare REQ_NOZERO
in a separate patch. This will tear off the flag declaration from the logic.

> Also, should we consider other code path used q->limits.max_write_zeroes_sectors?

Other code paths should not dereference q->limits.max_allocate_sectors, unless
it is directly set in not-zero value. In case of max_allocate_sectors is zero,
high-level primitives (generic_make_request_checks(), __blkdev_issue_write_zeroes(), ..)
complete such the bios immediately. Other drivers may need additional work
to support this, and really only subset of drivers need support of this, so this is
not a subject of this patchset.

Hm, it looks like there is an exception, which may inherit stack limits from children.
Device-mapper will pick all the limits we enable for children.
We may disable REQ_WRITE_ZEROES|REQ_NOZERO directly there, since it's not supported
in this driver yet.

Are you hinting at this here?

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 0a2cc197f62b..b8aa5f6f9ce1 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -489,6 +489,7 @@ static int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
 		       (unsigned long long) start << SECTOR_SHIFT);
 
 	limits->zoned = blk_queue_zoned_model(q);
+	limits->max_allocate_sectors = 0;
 
 	return 0;
 }
@@ -1548,6 +1549,7 @@ int dm_calculate_queue_limits(struct dm_table *table,
 			       dm_device_name(table->md),
 			       (unsigned long long) ti->begin,
 			       (unsigned long long) ti->len);
+		limits->max_allocate_sectors = 0;
 
 		/*
 		 * FIXME: this should likely be moved to blk_stack_limits(), would

Thanks,
Kirill