Re: [PATCH v6 4/6] block: Add support for REQ_ALLOCATE flag

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 12.02.2020 21:31, Darrick J. Wong wrote:
> On Wed, Feb 12, 2020 at 08:33:52PM +0300, Kirill Tkhai wrote:
>> Hi, Darrick,
>>
>> On 12.02.2020 19:58, Darrick J. Wong wrote:
>>> On Mon, Feb 10, 2020 at 12:33:53PM +0300, Kirill Tkhai wrote:
>>>> This adds support for REQ_ALLOCATE 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_ALLOCATE 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().
>>>>
>>>> Stacking devices start from zero max_allocate_sectors limit for now,
>>>> and the support is going to be implemented separate for each device
>>>> in the future.
>>>>
>>>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>>>> Reviewed-by: Bob Liu <bob.liu@oracle.com>
>>>> ---
>>>>  block/blk-lib.c           |   17 ++++++++++-------
>>>>  block/blk-settings.c      |    4 ++++
>>>>  fs/block_dev.c            |    4 ++++
>>>>  include/linux/blk_types.h |    5 ++++-
>>>>  include/linux/blkdev.h    |   13 ++++++++++---
>>>>  5 files changed, 32 insertions(+), 11 deletions(-)
>>>>
>>>> diff --git a/block/blk-lib.c b/block/blk-lib.c
>>>> index 3e38c93cfc53..9cd6f86523ba 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_ALLOCATE|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-settings.c b/block/blk-settings.c
>>>> index c8eda2e7b91e..8d5df9d37239 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 = 0;
>>>>  }
>>>>  EXPORT_SYMBOL(blk_set_stacking_limits);
>>>>  
>>>> @@ -506,6 +508,8 @@ 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);
>>>
>>> I think this should be ^^^ indented to match the other calls.
>>
>> The only idea I have about this is something like the below. But the below is over 90 char...
>>
>> 		error = blkdev_issue_zeroout(bdev, start >> 9, len >> 9,
>> 					     GFP_KERNEL,
>> 					     BLKDEV_ZERO_ALLOCATE | BLKDEV_ZERO_NOFALLBACK);
>>
>> Could you please clarify what you mean?
> 
> I mostly meant that the indent for the nth lines ought to be more than a
> single indent to make it easier to scan through the code, but you're
> right, the kernel indentation style is uglier.  I could suggest
> something like this, which actually does fit:
> 
> 	case FALLOC_FL_KEEP_SIZE:
> 		error = blkdev_issue_zeroout(bdev, start >> 9, len >> 9,
> 				GFP_KERNEL,
> 				BLKDEV_ZERO_ALLOCATE | BLKDEV_ZERO_NOFALLBACK);
> 		break;
> 
> But that's apparently apocryphal. :/

Just to mention.

We could also introduce BLKDEV_ZERO_ALLOCATE_NOFALLBACK, which is combination
of both bits. But this does not look good for me, since this make grep over
kernel code more difficult. And it won't good to introduce a new identifier
for a single place.

> --D
> 
>>>>  	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..86accd2caa4e 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_ALLOCATE,		/* only notify about allocated blocks,
>>>> +				 * and do not actually zero them
>>>
>>> "only notify"?  Is someone getting a notification?  Or are we simply
>>> "notifying" the device that it must ensure allocated blocks?
>>>
>>> If it's that last one, then perhaps this should be reworded:
>>>
>>> /*
>>>  * Ensure the LBA range is backed by physical storage
>>>  * without writing zeroes to the blocks.
>>>  */
>>
>> Sounds good.
>>  
>>>> +				 */
>>>>  	__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_ALLOCATE		(1ULL << __REQ_ALLOCATE)
>>>>  #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 55a714161684..40707f980a2e 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;
>>>>  
>>>> @@ -990,6 +991,8 @@ static inline struct bio_vec req_bvec(struct request *rq)
>>>>  static inline unsigned int blk_queue_get_max_write_zeroes_sectors(
>>>>  		struct request_queue *q, unsigned int op_flags)
>>>>  {
>>>> +	if (op_flags & REQ_ALLOCATE)
>>>> +		return q->limits.max_allocate_sectors;
>>>>  	return q->limits.max_write_zeroes_sectors;
>>>>  }
>>>>  
>>>> @@ -1226,6 +1229,7 @@ extern int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>>>>  
>>>>  #define BLKDEV_ZERO_NOUNMAP	(1 << 0)  /* do not free blocks */
>>>>  #define BLKDEV_ZERO_NOFALLBACK	(1 << 1)  /* don't write explicit zeroes */
>>>> +#define BLKDEV_ZERO_ALLOCATE	(1 << 2)  /* allocate range of blocks */
>>>>  
>>>>  extern int __blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
>>>>  		sector_t nr_sects, gfp_t gfp_mask, struct bio **biop,
>>>> @@ -1430,10 +1434,13 @@ static inline unsigned int bdev_write_zeroes_sectors(struct block_device *bdev,
>>>>  {
>>>>  	struct request_queue *q = bdev_get_queue(bdev);
>>>>  
>>>> -	if (q)
>>>> -		return q->limits.max_write_zeroes_sectors;
>>>> +	if (!q)
>>>> +		return 0;
>>>>  
>>>> -	return 0;
>>>> +	if (flags & BLKDEV_ZERO_ALLOCATE)
>>>> +		return q->limits.max_allocate_sectors;
>>>> +	else
>>>> +		return q->limits.max_write_zeroes_sectors;
>>>>  }
>>>>  
>>>>  static inline enum blk_zoned_model bdev_zoned_model(struct block_device *bdev)
>>>>
>>>>
>>
>> Thanks,
>> Kirill