Re: [PATCH 2/2] btrfs: add ioctl for directly writing compressed data

[Nikolay Borisov <nborisov@suse.com>]

On 4.09.19 г. 22:13 ч., Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> This adds an API for writing compressed data directly to the filesystem.
> The use case that I have in mind is send/receive: currently, when
> sending data from one compressed filesystem to another, the sending side
> decompresses the data and the receiving side recompresses it before
> writing it out. This is wasteful and can be avoided if we can just send
> and write compressed extents. The send part will be implemented in a
> separate series, as this ioctl can stand alone.
> 
> The interface is essentially pwrite(2) with some extra information:
> 
> - The input buffer contains the compressed data.
> - Both the compressed and decompressed sizes of the data are given.
> - The compression type (zlib, lzo, or zstd) is given.
> 
> The interface is general enough that it can be extended to encrypted or
> otherwise encoded extents in the future. A more detailed description,
> including restrictions and edge cases, is included in
> include/uapi/linux/btrfs.h.
> 
> The implementation is similar to direct I/O: we have to flush any
> ordered extents, invalidate the page cache, and do the io
> tree/delalloc/extent map/ordered extent dance. From there, we can reuse
> the compression code with a minor modification to distinguish the new
> ioctl from writeback.
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>


Should we choose to continue with this interface (based on Dave's
feedback) I'd rather see the following things changed:

> ---
>  fs/btrfs/compression.c     |   6 +-
>  fs/btrfs/compression.h     |  14 +--
>  fs/btrfs/ctree.h           |   6 ++
>  fs/btrfs/file.c            |  13 ++-
>  fs/btrfs/inode.c           | 192 ++++++++++++++++++++++++++++++++++++-
>  fs/btrfs/ioctl.c           |  95 ++++++++++++++++++
>  include/uapi/linux/btrfs.h |  69 +++++++++++++
>  7 files changed, 380 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
> index b05b361e2062..6632dd8d2e4d 100644
> --- a/fs/btrfs/compression.c
> +++ b/fs/btrfs/compression.c
> @@ -276,7 +276,8 @@ static void end_compressed_bio_write(struct bio *bio)
>  			bio->bi_status == BLK_STS_OK);
>  	cb->compressed_pages[0]->mapping = NULL;
>  
> -	end_compressed_writeback(inode, cb);
> +	if (cb->writeback)
> +		end_compressed_writeback(inode, cb);
>  	/* note, our inode could be gone now */
>  
>  	/*
> @@ -311,7 +312,7 @@ blk_status_t btrfs_submit_compressed_write(struct inode *inode, u64 start,
>  				 unsigned long compressed_len,
>  				 struct page **compressed_pages,
>  				 unsigned long nr_pages,
> -				 unsigned int write_flags)
> +				 unsigned int write_flags, bool writeback)
>  {
>  	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
>  	struct bio *bio = NULL;
> @@ -336,6 +337,7 @@ blk_status_t btrfs_submit_compressed_write(struct inode *inode, u64 start,
>  	cb->mirror_num = 0;
>  	cb->compressed_pages = compressed_pages;
>  	cb->compressed_len = compressed_len;
> +	cb->writeback = writeback;
>  	cb->orig_bio = NULL;
>  	cb->nr_pages = nr_pages;
>  
> diff --git a/fs/btrfs/compression.h b/fs/btrfs/compression.h
> index 4cb8be9ff88b..5b48eb730362 100644
> --- a/fs/btrfs/compression.h
> +++ b/fs/btrfs/compression.h
> @@ -6,6 +6,7 @@
>  #ifndef BTRFS_COMPRESSION_H
>  #define BTRFS_COMPRESSION_H
>  
> +#include <linux/btrfs.h>
>  #include <linux/sizes.h>
>  
>  /*
> @@ -47,6 +48,9 @@ struct compressed_bio {
>  	/* the compression algorithm for this bio */
>  	int compress_type;
>  
> +	/* Whether this is a write for writeback. */
> +	bool writeback;
> +
>  	/* number of compressed pages in the array */
>  	unsigned long nr_pages;
>  
> @@ -93,20 +97,12 @@ blk_status_t btrfs_submit_compressed_write(struct inode *inode, u64 start,
>  				  unsigned long compressed_len,
>  				  struct page **compressed_pages,
>  				  unsigned long nr_pages,
> -				  unsigned int write_flags);
> +				  unsigned int write_flags, bool writeback);
>  blk_status_t btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
>  				 int mirror_num, unsigned long bio_flags);
>  
>  unsigned int btrfs_compress_str2level(unsigned int type, const char *str);
>  
> -enum btrfs_compression_type {
> -	BTRFS_COMPRESS_NONE  = 0,
> -	BTRFS_COMPRESS_ZLIB  = 1,
> -	BTRFS_COMPRESS_LZO   = 2,
> -	BTRFS_COMPRESS_ZSTD  = 3,
> -	BTRFS_COMPRESS_TYPES = 3,
> -};
> -
>  struct workspace_manager {
>  	const struct btrfs_compress_op *ops;
>  	struct list_head idle_ws;
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 19d669d12ca1..9fae9b3f1f62 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -2905,6 +2905,10 @@ int btrfs_run_delalloc_range(struct inode *inode, struct page *locked_page,
>  int btrfs_writepage_cow_fixup(struct page *page, u64 start, u64 end);
>  void btrfs_writepage_endio_finish_ordered(struct page *page, u64 start,
>  					  u64 end, int uptodate);
> +
> +ssize_t btrfs_raw_write(struct kiocb *iocb, struct iov_iter *from,
> +			struct btrfs_ioctl_raw_pwrite_args *raw);
> +
>  extern const struct dentry_operations btrfs_dentry_operations;
>  
>  /* ioctl.c */
> @@ -2928,6 +2932,8 @@ int btrfs_add_inode_defrag(struct btrfs_trans_handle *trans,
>  			   struct btrfs_inode *inode);
>  int btrfs_run_defrag_inodes(struct btrfs_fs_info *fs_info);
>  void btrfs_cleanup_defrag_inodes(struct btrfs_fs_info *fs_info);
> +ssize_t btrfs_do_write_iter(struct kiocb *iocb, struct iov_iter *from,
> +			    struct btrfs_ioctl_raw_pwrite_args *args);
>  int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync);
>  void btrfs_drop_extent_cache(struct btrfs_inode *inode, u64 start, u64 end,
>  			     int skip_pinned);
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 8fe4eb7e5045..ed23aa65b2d5 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1872,8 +1872,8 @@ static void update_time_for_write(struct inode *inode)
>  		inode_inc_iversion(inode);
>  }
>  
> -static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
> -				    struct iov_iter *from)
> +ssize_t btrfs_do_write_iter(struct kiocb *iocb, struct iov_iter *from,
> +			    struct btrfs_ioctl_raw_pwrite_args *raw)
>  {
>  	struct file *file = iocb->ki_filp;
>  	struct inode *inode = file_inode(file);
> @@ -1965,7 +1965,9 @@ 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 (raw) {
> +		num_written = btrfs_raw_write(iocb, from, raw);
> +	} else if (iocb->ki_flags & IOCB_DIRECT) {
>  		num_written = __btrfs_direct_write(iocb, from);
>  	} else {
>  		num_written = btrfs_buffered_write(iocb, from);
> @@ -1996,6 +1998,11 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
>  	return num_written ? num_written : err;
>  }
>  
> +static ssize_t btrfs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
> +{
> +	return btrfs_do_write_iter(iocb, from, NULL);
> +}
> +
>  int btrfs_release_file(struct inode *inode, struct file *filp)
>  {
>  	struct btrfs_file_private *private = filp->private_data;
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index a0546401bc0a..c8eaa1e5bf06 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -865,7 +865,7 @@ static noinline void submit_compressed_extents(struct async_chunk *async_chunk)
>  				    ins.objectid,
>  				    ins.offset, async_extent->pages,
>  				    async_extent->nr_pages,
> -				    async_chunk->write_flags)) {
> +				    async_chunk->write_flags, true)) {
>  			struct page *p = async_extent->pages[0];
>  			const u64 start = async_extent->start;
>  			const u64 end = start + async_extent->ram_size - 1;
> @@ -10590,6 +10590,196 @@ void btrfs_set_range_writeback(struct extent_io_tree *tree, u64 start, u64 end)
>  	}
>  }
>  
> +/* Currently, this only supports raw writes of compressed data. */
> +ssize_t btrfs_raw_write(struct kiocb *iocb, struct iov_iter *from,
> +			struct btrfs_ioctl_raw_pwrite_args *raw)
> +{
> +	struct file *file = iocb->ki_filp;
> +	struct inode *inode = file_inode(file);
> +	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
> +	struct btrfs_root *root = BTRFS_I(inode)->root;
> +	struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
> +	struct extent_changeset *data_reserved = NULL;
> +	struct extent_state *cached_state = NULL;
> +	unsigned long nr_pages, i;
> +	struct page **pages;
> +	u64 disk_num_bytes, num_bytes;
> +	u64 start, end;
> +	struct btrfs_key ins;
> +	struct extent_map *em;
> +	ssize_t ret;
> +
> +	if (iov_iter_count(from) != raw->num_bytes) {
> +		/*
> +		 * The write got truncated by generic_write_checks(). We can't
> +		 * do a partial raw write.
> +		 */
> +		return -EFBIG;
> +	}
> +
> +	/* This should be handled higher up. */
> +	ASSERT(raw->num_bytes != 0);

This is already checked, indirectly via rw_verify_area, there's :

 if (unlikely((ssize_t) count < 0))
                  return retval;

So you can remove this assert.

> +
> +	/* The extent size must be sane. */
> +	if (raw->num_bytes > BTRFS_MAX_UNCOMPRESSED ||
> +	    raw->disk_num_bytes > BTRFS_MAX_COMPRESSED ||
> +	    raw->disk_num_bytes == 0)
> +		return -EINVAL;
> +
> +	/*
> +	 * The compressed data on disk must be sector-aligned. For convenience,
> +	 * we extend the compressed data with zeroes if it isn't.
> +	 */
> +	disk_num_bytes = ALIGN(raw->disk_num_bytes, fs_info->sectorsize);
> +	/*
> +	 * The extent in the file must also be sector-aligned. However, we allow
> +	 * a write which ends at or extends i_size to have an unaligned length;
> +	 * we round up the extent size and set i_size to the given length.
> +	 */
> +	start = iocb->ki_pos;
> +	if ((start & (fs_info->sectorsize - 1)))

if (!IS_ALIGNED(start, fs_info->sectorsize))

> +		return -EINVAL;
> +	if (start + raw->num_bytes >= inode->i_size) {
> +		num_bytes = ALIGN(raw->num_bytes, fs_info->sectorsize);
> +	} else {
> +		num_bytes = raw->num_bytes;
> +		if ((num_bytes & (fs_info->sectorsize - 1)))

ditto

> +			return -EINVAL;
> +	}
> +	end = start + num_bytes - 1;
> +
> +	/*
> +	 * It's valid for compressed data to be larger than or the same size as
> +	 * the decompressed data. However, for buffered I/O, we never write out
> +	 * a compressed extent unless it's smaller than the decompressed data,
> +	 * so for now, let's not allow creating such extents with the ioctl,
> +	 * either.
> +	 */
> +	if (disk_num_bytes >= num_bytes)
> +		return -EINVAL;
> +
> +	nr_pages = DIV_ROUND_UP(disk_num_bytes, PAGE_SIZE);
> +	pages = kcalloc(nr_pages, sizeof(struct page *),
> +			GFP_USER | __GFP_NOWARN);
> +	if (!pages)
> +		return -ENOMEM;
> +	for (i = 0; i < nr_pages; i++) {
> +		unsigned long offset = i << PAGE_SHIFT, n;
> +		char *kaddr;
> +
> +		pages[i] = alloc_page(GFP_USER | __GFP_NOWARN);
> +		if (!pages[i]) {
> +			ret = -ENOMEM;
> +			goto out_pages;
> +		}
> +		kaddr = kmap(pages[i]);
> +		if (offset < raw->disk_num_bytes) {
> +			n = min_t(unsigned long, PAGE_SIZE,
> +				  raw->disk_num_bytes - offset);
> +			if (copy_from_user(kaddr, raw->buf + offset, n)) {
> +				kunmap(pages[i]);
> +				ret = -EFAULT;
> +				goto out_pages;
> +			}
> +		} else {
> +			n = 0;
> +		}
> +		if (n < PAGE_SIZE)
> +			memset(kaddr + n, 0, PAGE_SIZE - n);
> +		kunmap(pages[i]);
> +	}
> +
> +	for (;;) {
> +		struct btrfs_ordered_extent *ordered;
> +
> +		lock_extent_bits(io_tree, start, end, &cached_state);
> +		ordered = btrfs_lookup_ordered_range(BTRFS_I(inode), start,
> +						     end - start + 1);
> +		if (!ordered &&
> +		    !filemap_range_has_page(inode->i_mapping, start, end))
> +			break;
> +		if (ordered)
> +			btrfs_put_ordered_extent(ordered);
> +		unlock_extent_cached(&BTRFS_I(inode)->io_tree, start, end,
> +				     &cached_state);
> +		cond_resched();
> +		ret = btrfs_wait_ordered_range(inode, start, end);
> +		if (ret)
> +			goto out_pages;
> +		ret = invalidate_inode_pages2_range(inode->i_mapping,
> +						    start >> PAGE_SHIFT,
> +						    end >> PAGE_SHIFT);
> +		if (ret)
> +			goto out_pages;
> +	}

Won't btrfs_lock_and_flush_ordered_range suffice here? Perhaps call that
function + invalidate_inode_pages2_range ?

> +
> +	ret = btrfs_delalloc_reserve_space(inode, &data_reserved, start,
> +					   num_bytes);
> +	if (ret)
> +		goto out_unlock;
> +
> +	ret = btrfs_reserve_extent(root, num_bytes, disk_num_bytes,
> +				   disk_num_bytes, 0, 0, &ins, 1, 1);
> +	if (ret)
> +		goto out_delalloc_release;
> +
> +	em = create_io_em(inode, start, num_bytes, start, ins.objectid,
> +			  ins.offset, ins.offset, num_bytes, raw->compression,
> +			  BTRFS_ORDERED_COMPRESSED);
> +	if (IS_ERR(em)) {
> +		ret = PTR_ERR(em);
> +		goto out_free_reserve;
> +	}
> +	free_extent_map(em);
> +
> +	ret = btrfs_add_ordered_extent_compress(inode, start, ins.objectid,
> +						num_bytes, ins.offset,
> +						BTRFS_ORDERED_COMPRESSED,
> +						raw->compression);
> +	if (ret) {
> +		btrfs_drop_extent_cache(BTRFS_I(inode), start, end, 0);
> +		goto out_free_reserve;
> +	}
> +	btrfs_dec_block_group_reservations(fs_info, ins.objectid);
> +
> +	if (start + raw->num_bytes > inode->i_size)
> +		i_size_write(inode, start + raw->num_bytes);
> +
> +	unlock_extent_cached(io_tree, start, end, &cached_state);
> +
> +	btrfs_delalloc_release_extents(BTRFS_I(inode), num_bytes, false);
> +
> +	if (btrfs_submit_compressed_write(inode, start, num_bytes, ins.objectid,
> +					  ins.offset, pages, nr_pages, 0,
> +					  false)) {
> +		struct page *page = pages[0];
> +
> +		page->mapping = inode->i_mapping;
> +		btrfs_writepage_endio_finish_ordered(page, start, end, 0);
> +		page->mapping = NULL;
> +		ret = -EIO;
> +		goto out_pages;
> +	}
> +	iocb->ki_pos += raw->num_bytes;
> +	return raw->num_bytes;
> +
> +out_free_reserve:
> +	btrfs_dec_block_group_reservations(fs_info, ins.objectid);
> +	btrfs_free_reserved_extent(fs_info, ins.objectid, ins.offset, 1);
> +out_delalloc_release:
> +	btrfs_delalloc_release_space(inode, data_reserved, start, num_bytes,
> +				     true);
> +out_unlock:
> +	unlock_extent_cached(io_tree, start, end, &cached_state);
> +out_pages:
> +	for (i = 0; i < nr_pages; i++) {
> +		if (pages[i])
> +			put_page(pages[i]);
> +	}
> +	kfree(pages);
> +	return ret;
> +}
> +
>  #ifdef CONFIG_SWAP
>  /*
>   * Add an entry indicating a block group or device which is pinned by a

<snip>