Re: [PATCH 07/18] btrfs: add dax write support

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

On Mon, Apr 29, 2019 at 12:26:38PM -0500, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> IOMAP_DAX_COW allows to inform the dax code, to first perform
> a copy which are not page-aligned before performing the write.
> The responsibility of checking if data edges are page aligned
> is performed in ->iomap_begin() and the source address is
> stored in ->inline_data
> 
> A new struct btrfs_iomap is passed from iomap_begin() to
> iomap_end(), which contains all the accounting and locking information
> for CoW based writes.
> 
> For writing to a hole, iomap->inline_data is set to zero.
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>  fs/btrfs/ctree.h |   6 ++
>  fs/btrfs/dax.c   | 182 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  fs/btrfs/file.c  |   4 +-
>  3 files changed, 185 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 1e3e758b83c2..eec01eb92f33 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -3801,6 +3801,12 @@ int btree_readahead_hook(struct extent_buffer *eb, int err);
>  #ifdef CONFIG_FS_DAX
>  /* dax.c */
>  ssize_t btrfs_file_dax_read(struct kiocb *iocb, struct iov_iter *to);
> +ssize_t btrfs_file_dax_write(struct kiocb *iocb, struct iov_iter *from);
> +#else
> +static inline ssize_t btrfs_file_dax_write(struct kiocb *iocb, struct iov_iter *from)
> +{
> +	return 0;
> +}
>  #endif /* CONFIG_FS_DAX */
>  
>  static inline int is_fstree(u64 rootid)
> diff --git a/fs/btrfs/dax.c b/fs/btrfs/dax.c
> index bf3d46b0acb6..f5cc9bcdbf14 100644
> --- a/fs/btrfs/dax.c
> +++ b/fs/btrfs/dax.c
> @@ -9,30 +9,184 @@
>  #ifdef CONFIG_FS_DAX
>  #include <linux/dax.h>
>  #include <linux/iomap.h>
> +#include <linux/uio.h>
>  #include "ctree.h"
>  #include "btrfs_inode.h"
>  
> +struct btrfs_iomap {
> +	u64 start;
> +	u64 end;
> +	bool nocow;
> +	struct extent_changeset *data_reserved;
> +	struct extent_state *cached_state;
> +};
> +
> +static struct btrfs_iomap *btrfs_iomap_init(struct inode *inode,
> +				     struct extent_map **em,
> +				     loff_t pos, loff_t length)
> +{
> +	int ret = 0;
> +	struct extent_map *map = *em;
> +	struct btrfs_iomap *bi;
> +
> +	bi = kzalloc(sizeof(struct btrfs_iomap), GFP_NOFS);
> +	if (!bi)
> +		return ERR_PTR(-ENOMEM);
> +
> +	bi->start = round_down(pos, PAGE_SIZE);
> +	bi->end = PAGE_ALIGN(pos + length);
> +
> +	/* Wait for existing ordered extents in range to finish */
> +	btrfs_wait_ordered_range(inode, bi->start, bi->end - bi->start);
> +
> +	lock_extent_bits(&BTRFS_I(inode)->io_tree, bi->start, bi->end, &bi->cached_state);
> +
> +	ret = btrfs_delalloc_reserve_space(inode, &bi->data_reserved,
> +			bi->start, bi->end - bi->start);
> +	if (ret) {
> +		unlock_extent_cached(&BTRFS_I(inode)->io_tree, bi->start, bi->end,
> +				&bi->cached_state);
> +		kfree(bi);
> +		return ERR_PTR(ret);
> +	}
> +
> +	refcount_inc(&map->refs);
> +	ret = btrfs_get_extent_map_write(em, NULL,
> +			inode, bi->start, bi->end - bi->start, &bi->nocow);
> +	if (ret) {
> +		unlock_extent_cached(&BTRFS_I(inode)->io_tree, bi->start, bi->end,
> +				&bi->cached_state);
> +		btrfs_delalloc_release_space(inode,
> +				bi->data_reserved, bi->start,
> +				bi->end - bi->start, true);
> +		extent_changeset_free(bi->data_reserved);
> +		kfree(bi);
> +		return ERR_PTR(ret);
> +	}
> +	free_extent_map(map);
> +	return bi;
> +}
> +
> +static void *dax_address(struct block_device *bdev, struct dax_device *dax_dev,
> +			 sector_t sector, loff_t pos, loff_t length)

This looks like a common function for fs/dax.c.

--D

> +{
> +	size_t size = ALIGN(pos + length, PAGE_SIZE);
> +	int id, ret = 0;
> +	void *kaddr = NULL;
> +	pgoff_t pgoff;
> +	long map_len;
> +
> +	id = dax_read_lock();
> +
> +	ret = bdev_dax_pgoff(bdev, sector, size, &pgoff);
> +	if (ret)
> +		goto out;
> +
> +	map_len = dax_direct_access(dax_dev, pgoff, PHYS_PFN(size),
> +			&kaddr, NULL);
> +	if (map_len < 0)
> +		ret = map_len;
> +
> +out:
> +	dax_read_unlock(id);
> +	if (ret)
> +		return ERR_PTR(ret);
> +	return kaddr;
> +}
> +
>  static int btrfs_iomap_begin(struct inode *inode, loff_t pos,
>  		loff_t length, unsigned flags, struct iomap *iomap)
>  {
>  	struct extent_map *em;
>  	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
> +	struct btrfs_iomap *bi = NULL;
> +	unsigned offset = pos & (PAGE_SIZE - 1);
> +	u64 srcblk = 0;
> +	loff_t diff;
> +
>  	em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, pos, length, 0);
> +
> +	iomap->type = IOMAP_MAPPED;
> +
> +	if (flags & IOMAP_WRITE) {
> +		if (em->block_start != EXTENT_MAP_HOLE)
> +			srcblk = em->block_start + pos - em->start - offset;
> +
> +		bi = btrfs_iomap_init(inode, &em, pos, length);
> +		if (IS_ERR(bi))
> +			return PTR_ERR(bi);
> +
> +	}
> +
> +	/*
> +	 * Advance the difference between pos and start, to align well with
> +	 * inline_data in case of writes
> +	 */
> +	diff = round_down(pos - em->start, PAGE_SIZE);
> +	iomap->offset = em->start + diff;
> +	iomap->length = em->len - diff;
> +	iomap->bdev = em->bdev;
> +	iomap->dax_dev = fs_info->dax_dev;
> +
> +	/*
> +	 * This will be true for reads only since we have already
> +	 * allocated em
> +	 */
>  	if (em->block_start == EXTENT_MAP_HOLE) {
>  		iomap->type = IOMAP_HOLE;
>  		return 0;
>  	}
> -	iomap->type = IOMAP_MAPPED;
> -	iomap->bdev = em->bdev;
> -	iomap->dax_dev = fs_info->dax_dev;
> -	iomap->offset = em->start;
> -	iomap->length = em->len;
> -	iomap->addr = em->block_start;
> +
> +	iomap->addr = em->block_start + diff;
> +	/* Check if we really need to copy data from old extent */
> +	if (bi && !bi->nocow && (offset || pos + length < bi->end)) {
> +		iomap->type = IOMAP_DAX_COW;
> +		if (srcblk) {
> +			sector_t sector = (srcblk + (pos & PAGE_MASK) -
> +					  iomap->offset) >> 9;
> +			iomap->inline_data = dax_address(em->bdev,
> +					fs_info->dax_dev, sector, pos, length);
> +			if (IS_ERR(iomap->inline_data)) {
> +				kfree(bi);
> +				return PTR_ERR(iomap->inline_data);
> +			}
> +		}
> +	}
> +
> +	iomap->private = bi;
> +	return 0;
> +}
> +
> +static int btrfs_iomap_end(struct inode *inode, loff_t pos,
> +		loff_t length, ssize_t written, unsigned flags,
> +		struct iomap *iomap)
> +{
> +	struct btrfs_iomap *bi = iomap->private;
> +	u64 wend;
> +
> +	if (!bi)
> +		return 0;
> +
> +	unlock_extent_cached(&BTRFS_I(inode)->io_tree, bi->start, bi->end,
> +			&bi->cached_state);
> +
> +	wend = PAGE_ALIGN(pos + written);
> +	if (wend < bi->end) {
> +		btrfs_delalloc_release_space(inode,
> +				bi->data_reserved, wend,
> +				bi->end - wend, true);
> +	}
> +
> +	btrfs_update_ordered_extent(inode, bi->start, wend - bi->start, true);
> +	btrfs_delalloc_release_extents(BTRFS_I(inode), wend - bi->start, false);
> +	extent_changeset_free(bi->data_reserved);
> +	kfree(bi);
>  	return 0;
>  }
>  
>  static const struct iomap_ops btrfs_iomap_ops = {
>  	.iomap_begin		= btrfs_iomap_begin,
> +	.iomap_end		= btrfs_iomap_end,
>  };
>  
>  ssize_t btrfs_file_dax_read(struct kiocb *iocb, struct iov_iter *to)
> @@ -46,4 +200,20 @@ ssize_t btrfs_file_dax_read(struct kiocb *iocb, struct iov_iter *to)
>  
>  	return ret;
>  }
> +
> +ssize_t btrfs_file_dax_write(struct kiocb *iocb, struct iov_iter *iter)
> +{
> +	ssize_t ret = 0;
> +	u64 pos = iocb->ki_pos;
> +	struct inode *inode = file_inode(iocb->ki_filp);
> +	ret = dax_iomap_rw(iocb, iter, &btrfs_iomap_ops);
> +
> +	if (ret > 0) {
> +		pos += ret;
> +		if (pos > i_size_read(inode))
> +			i_size_write(inode, pos);
> +		iocb->ki_pos = pos;
> +	}
> +	return ret;
> +}
>  #endif /* CONFIG_FS_DAX */
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 9194591f9eea..a795023e26ca 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1964,7 +1964,9 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
>  	if (sync)
>  		atomic_inc(&BTRFS_I(inode)->sync_writers);
>  
> -	if (iocb->ki_flags & IOCB_DIRECT) {
> +	if (IS_DAX(inode)) {
> +		num_written = btrfs_file_dax_write(iocb, from);
> +	} else if (iocb->ki_flags & IOCB_DIRECT) {
>  		num_written = __btrfs_direct_write(iocb, from);
>  	} else {
>  		num_written = btrfs_buffered_write(iocb, from);
> -- 
> 2.16.4
>