Re: [RFC PATCH] block: wire blkdev_fallocate() to block_device_operations' reserve_space

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Apr 12, 2016 at 04:04:59PM -0400, Mike Snitzer wrote:
> On Tue, Apr 12 2016 at 12:42P -0400,
> Brian Foster <bfoster@redhat.com> wrote:
> 
> > Hi all,
> > 
> > This is v2 of the XFS and block device reservation experiment. The
> > significant changes in v2 are that the bdev interface has been condensed
> > to a single callback function, the XFS transaction reservation
> > management has been reworked to make transactions responsible for
> > tracking and releasing excess reservation (for non-delalloc cases) and a
> > workaround for the fallocate over-reservation issue is included. Beyond
> > that, this version adds a bunch of miscellaneous cleanups and fixes some
> > of the nastier locking/leak issues present in the first rfc.
> > 
> > Patches 1-2 refactor some XFS reserve pool and block accounting code in
> > preparation for subsequent patches. Patches 3-5 add block/device-mapper
> > reservation support. Patches 6-10 add the core reservation
> > infrastructure and management bits to XFS. See the link to the original
> > rfc below for instructions and further details around the purpose of
> > this series.
> > 
> > Finally, note that this is still highly experimental/theoretical and
> > should not be used on production systems. Thoughts, reviews, flames
> > appreciated.
> 
> Thanks for carrying on with this work Brian.
> 
> I've started to review your patchset and Darrick's fallocate patchset.
> I've pushed a branch to linux-dm.git that combines the 2, see:
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-fallocate
> 
> and then added this RFC patch, at the end, which relies on both of your
> patchsets -- you'll see blkdev_ensure_space_exists() has a FIXME which
> implies it isn't much more than simply stubbed out at this point
> (completely untested):

Hmm, ok, but -rc3 broke a bunch of stuff.  Guess I should repost with all
the PAGE_CACHE_ -> PAGE_ stuff fixed. :)

> From: Mike Snitzer <snitzer@redhat.com>
> Date: Tue, 12 Apr 2016 15:54:31 -0400
> Subject: [RFC PATCH] block: wire blkdev_fallocate() to block_device_operations' reserve_space
> 
> This effectively exposes the primitive for "ensure space exists".  It
> relies on block_device_operations' reserve_space method.
> 
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> ---
>  block/blk-lib.c        | 26 ++++++++++++++++++++++++++
>  fs/block_dev.c         | 20 +++++++++++---------
>  include/linux/blkdev.h |  2 ++
>  3 files changed, 39 insertions(+), 9 deletions(-)
> 
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index 9dca6bb..5042a84 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -314,3 +314,29 @@ int blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
>  	return __blkdev_issue_zeroout(bdev, sector, nr_sects, gfp_mask);
>  }
>  EXPORT_SYMBOL(blkdev_issue_zeroout);
> +
> +/**
> + * blkdev_ensure_space_exists - preallocate a block range
> + * @bdev:	blockdev to preallocate space for
> + * @sector:	start sector
> + * @nr_sects:	number of sectors to preallocate
> + * @gfp_mask:	memory allocation flags (for bio_alloc)
> + * @flags:	FALLOC_FL_* to control behaviour
> + *
> + * Description:
> + *    Ensure space exists, or is preallocated, for the sectors in question.
> + */
> +int blkdev_ensure_space_exists(struct block_device *bdev, sector_t sector,
> +		sector_t nr_sects, unsigned long flags)
> +{
> +	sector_t res;
> +	const struct block_device_operations *ops = bdev->bd_disk->fops;
> +
> +	if (!ops->reserve_space)
> +		return -EOPNOTSUPP;
> +
> +	// FIXME: check with Brian Foster on whether it makes sense to
> +	// use BDEV_RES_GET/BDEV_RES_MOD instead of BDEV_RES_PROVISION?
> +	return ops->reserve_space(bdev, BDEV_RES_PROVISION, sector, nr_sects, &res);

/me thinks BDEV_RES_PROVISION is correct here, because regular-mode file
fallocate (for ext4/xfs anyway) allocates blocks and maps them to specific file
offsets as unwritten extents.  afaict RES_PROVISION -> thin_provision_space()
and thin_provision_space() seems to allocate blocks and map them to the
device's LBAs.

If I'm reading the patches correctly, RES_GET/RES_MOD seem to reserve N blocks
but doesn't map them to any specific LBA.

> +}
> +EXPORT_SYMBOL(blkdev_ensure_space_exists);
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 5a2c3ab..b34c07b 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1801,17 +1801,13 @@ long blkdev_fallocate(struct file *file, int mode, loff_t start, loff_t len)
>  	struct request_queue *q = bdev_get_queue(bdev);
>  	struct address_space *mapping;
>  	loff_t end = start + len - 1;
> -	loff_t bs_mask, isize;
> +	loff_t isize;
>  	int error;
>  
>  	/* We only support zero range and punch hole. */
>  	if (mode & ~BLKDEV_FALLOC_FL_SUPPORTED)
>  		return -EOPNOTSUPP;
>  
> -	/* We haven't a primitive for "ensure space exists" right now. */
> -	if (!(mode & ~FALLOC_FL_KEEP_SIZE))
> -		return -EOPNOTSUPP;
> -
>  	/* Only punch if the device can do zeroing discard. */
>  	if ((mode & FALLOC_FL_PUNCH_HOLE) &&
>  	    (!blk_queue_discard(q) || !q->limits.discard_zeroes_data))
> @@ -1829,9 +1825,12 @@ long blkdev_fallocate(struct file *file, int mode, loff_t start, loff_t len)
>  			return -EINVAL;
>  	}
>  
> -	/* Don't allow IO that isn't aligned to logical block size */
> -	bs_mask = bdev_logical_block_size(bdev) - 1;
> -	if ((start | len) & bs_mask)
> +	/*
> +	 * Don't allow IO that isn't aligned to minimum IO size (io_min)
> +	 * - for normal device's io_min is usually logical block size
> +	 * - but for more exotic devices (e.g. DM thinp) it may be larger
> +	 */
> +	if ((start | len) % bdev_io_min(bdev))
>  		return -EINVAL;

Noted.  Will update the original patch.

>  	/* Invalidate the page cache, including dirty pages. */
> @@ -1839,7 +1838,10 @@ long blkdev_fallocate(struct file *file, int mode, loff_t start, loff_t len)
>  	truncate_inode_pages_range(mapping, start, end);
>  
>  	error = -EINVAL;
> -	if (mode & FALLOC_FL_ZERO_RANGE)
> +	if (!(mode & ~FALLOC_FL_KEEP_SIZE))
> +		error = blkdev_ensure_space_exists(bdev, start >> 9, len >> 9,
> +						   mode);
> +	else if (mode & FALLOC_FL_ZERO_RANGE)

This whole thing got converted to a switch statement due to some feedback
from hch.

Anyway, will try to have a new blockdev fallocate patchset done by the end
of the day.

(Is there a test case for this?)

--D

>  		error = blkdev_issue_zeroout(bdev, start >> 9, len >> 9,
>  					    GFP_KERNEL, false);
>  	else if (mode & FALLOC_FL_PUNCH_HOLE)
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 6c6ea96..4147af2 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1132,6 +1132,8 @@ extern int blkdev_issue_write_same(struct block_device *bdev, sector_t sector,
>  		sector_t nr_sects, gfp_t gfp_mask, struct page *page);
>  extern int blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
>  		sector_t nr_sects, gfp_t gfp_mask, bool discard);
> +extern int blkdev_ensure_space_exists(struct block_device *bdev, sector_t sector,
> +		sector_t nr_sects, unsigned long flags);
>  static inline int sb_issue_discard(struct super_block *sb, sector_t block,
>  		sector_t nr_blocks, gfp_t gfp_mask, unsigned long flags)
>  {
> -- 
> 2.6.4 (Apple Git-63)
> 
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