Re: [PATCH 02/18] iomap: add initial support for writes without buffer heads

[Brian Foster <bfoster@redhat.com>]

On Wed, May 30, 2018 at 11:59:57AM +0200, Christoph Hellwig wrote:
> For now just limited to blocksize == PAGE_SIZE, where we can simply read
> in the full page in write begin, and just set the whole page dirty after
> copying data into it.  This code is enabled by default and XFS will now
> be feed pages without buffer heads in ->writepage and ->writepages.
> 
> If a file system sets the IOMAP_F_BUFFER_HEAD flag on the iomap the old
> path will still be used, this both helps the transition in XFS and
> prepares for the gfs2 migration to the iomap infrastructure.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  fs/iomap.c            | 128 ++++++++++++++++++++++++++++++++++++++----
>  fs/xfs/xfs_iomap.c    |   6 +-
>  include/linux/iomap.h |   2 +
>  3 files changed, 123 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/iomap.c b/fs/iomap.c
> index 5e5a266e3325..0c9d9be59184 100644
> --- a/fs/iomap.c
> +++ b/fs/iomap.c
> @@ -316,6 +316,48 @@ iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
>  		truncate_pagecache_range(inode, max(pos, i_size), pos + len);
>  }
>  
> +static int
> +iomap_read_page_sync(struct inode *inode, loff_t block_start, struct page *page,
> +		unsigned poff, unsigned plen, unsigned from, unsigned to,
> +		struct iomap *iomap)
> +{
> +	struct bio_vec bvec;
> +	struct bio bio;
> +
> +	if (iomap->type != IOMAP_MAPPED || block_start >= i_size_read(inode)) {
> +		zero_user_segments(page, poff, from, to, poff + plen);
> +		return 0;
> +	}
> +
> +	bio_init(&bio, &bvec, 1);
> +	bio.bi_opf = REQ_OP_READ;
> +	bio.bi_iter.bi_sector = iomap_sector(iomap, block_start);
> +	bio_set_dev(&bio, iomap->bdev);
> +	__bio_add_page(&bio, page, plen, poff);
> +	return submit_bio_wait(&bio);
> +}
> +
> +static int
> +__iomap_write_begin(struct inode *inode, loff_t pos, unsigned len,
> +		struct page *page, struct iomap *iomap)
> +{
> +	loff_t block_size = i_blocksize(inode);
> +	loff_t block_start = pos & ~(block_size - 1);
> +	loff_t block_end = (pos + len + block_size - 1) & ~(block_size - 1);
> +	unsigned poff = block_start & (PAGE_SIZE - 1);
> +	unsigned plen = min_t(loff_t, PAGE_SIZE - poff, block_end - block_start);
> +	unsigned from = pos & (PAGE_SIZE - 1), to = from + len;
> +
> +	WARN_ON_ONCE(i_blocksize(inode) < PAGE_SIZE);
> +
> +	if (PageUptodate(page))
> +		return 0;
> +	if (from <= poff && to >= poff + plen)
> +		return 0;
> +	return iomap_read_page_sync(inode, block_start, page,
> +			poff, plen, from, to, iomap);
> +}
> +
>  static int
>  iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>  		struct page **pagep, struct iomap *iomap)
> @@ -333,7 +375,10 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>  	if (!page)
>  		return -ENOMEM;
>  
> -	status = __block_write_begin_int(page, pos, len, NULL, iomap);
> +	if (iomap->flags & IOMAP_F_BUFFER_HEAD)
> +		status = __block_write_begin_int(page, pos, len, NULL, iomap);
> +	else
> +		status = __iomap_write_begin(inode, pos, len, page, iomap);
>  	if (unlikely(status)) {
>  		unlock_page(page);
>  		put_page(page);
> @@ -346,14 +391,69 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>  	return status;
>  }
>  
> +int
> +iomap_set_page_dirty(struct page *page)
> +{
> +	struct address_space *mapping = page_mapping(page);
> +	int newly_dirty;
> +
> +	if (unlikely(!mapping))
> +		return !TestSetPageDirty(page);
> +
> +	/*
> +	 * Lock out page->mem_cgroup migration to keep PageDirty
> +	 * synchronized with per-memcg dirty page counters.
> +	 */
> +	lock_page_memcg(page);
> +	newly_dirty = !TestSetPageDirty(page);
> +	if (newly_dirty)
> +		__set_page_dirty(page, mapping, 0);
> +	unlock_page_memcg(page);
> +
> +	if (newly_dirty)
> +		__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
> +	return newly_dirty;
> +}
> +EXPORT_SYMBOL_GPL(iomap_set_page_dirty);
> +
> +static int
> +__iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
> +		unsigned copied, struct page *page, struct iomap *iomap)
> +{
> +	flush_dcache_page(page);
> +
> +	/*
> +	 * The blocks that were entirely written will now be uptodate, so we
> +	 * don't have to worry about a readpage reading them and overwriting a
> +	 * partial write.  However if we have encountered a short write and only
> +	 * partially written into a block, it will not be marked uptodate, so a
> +	 * readpage might come in and destroy our partial write.
> +	 *
> +	 * Do the simplest thing, and just treat any short write to a non
> +	 * uptodate page as a zero-length write, and force the caller to redo
> +	 * the whole thing.
> +	 */
> +	if (unlikely(copied < len && !PageUptodate(page))) {
> +		copied = 0;
> +	} else {
> +		SetPageUptodate(page);
> +		iomap_set_page_dirty(page);
> +	}
> +	return __generic_write_end(inode, pos, copied, page);
> +}
> +
>  static int
>  iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
> -		unsigned copied, struct page *page)
> +		unsigned copied, struct page *page, struct iomap *iomap)
>  {
>  	int ret;
>  
> -	ret = generic_write_end(NULL, inode->i_mapping, pos, len,
> -			copied, page, NULL);
> +	if (iomap->flags & IOMAP_F_BUFFER_HEAD)
> +		ret = generic_write_end(NULL, inode->i_mapping, pos, len,
> +				copied, page, NULL);
> +	else
> +		ret = __iomap_write_end(inode, pos, len, copied, page, iomap);
> +
>  	if (ret < len)
>  		iomap_write_failed(inode, pos, len);
>  	return ret;
> @@ -408,7 +508,8 @@ iomap_write_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  
>  		flush_dcache_page(page);
>  
> -		status = iomap_write_end(inode, pos, bytes, copied, page);
> +		status = iomap_write_end(inode, pos, bytes, copied, page,
> +				iomap);
>  		if (unlikely(status < 0))
>  			break;
>  		copied = status;
> @@ -502,7 +603,7 @@ iomap_dirty_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  
>  		WARN_ON_ONCE(!PageUptodate(page));
>  
> -		status = iomap_write_end(inode, pos, bytes, bytes, page);
> +		status = iomap_write_end(inode, pos, bytes, bytes, page, iomap);
>  		if (unlikely(status <= 0)) {
>  			if (WARN_ON_ONCE(status == 0))
>  				return -EIO;
> @@ -554,7 +655,7 @@ static int iomap_zero(struct inode *inode, loff_t pos, unsigned offset,
>  	zero_user(page, offset, bytes);
>  	mark_page_accessed(page);
>  
> -	return iomap_write_end(inode, pos, bytes, bytes, page);
> +	return iomap_write_end(inode, pos, bytes, bytes, page, iomap);
>  }
>  
>  static int iomap_dax_zero(loff_t pos, unsigned offset, unsigned bytes,
> @@ -640,11 +741,16 @@ iomap_page_mkwrite_actor(struct inode *inode, loff_t pos, loff_t length,
>  	struct page *page = data;
>  	int ret;
>  
> -	ret = __block_write_begin_int(page, pos, length, NULL, iomap);
> -	if (ret)
> -		return ret;
> +	if (iomap->flags & IOMAP_F_BUFFER_HEAD) {
> +		ret = __block_write_begin_int(page, pos, length, NULL, iomap);
> +		if (ret)
> +			return ret;
> +		block_commit_write(page, 0, length);
> +	} else {
> +		WARN_ON_ONCE(!PageUptodate(page));
> +		WARN_ON_ONCE(i_blocksize(inode) < PAGE_SIZE);
> +	}
>  
> -	block_commit_write(page, 0, length);
>  	return length;
>  }
>  
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index c6ce6f9335b6..da6d1995e460 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -638,7 +638,7 @@ xfs_file_iomap_begin_delay(
>  	 * Flag newly allocated delalloc blocks with IOMAP_F_NEW so we punch
>  	 * them out if the write happens to fail.
>  	 */
> -	iomap->flags = IOMAP_F_NEW;
> +	iomap->flags |= IOMAP_F_NEW;
>  	trace_xfs_iomap_alloc(ip, offset, count, 0, &got);
>  done:
>  	if (isnullstartblock(got.br_startblock))
> @@ -1031,6 +1031,8 @@ xfs_file_iomap_begin(
>  	if (XFS_FORCED_SHUTDOWN(mp))
>  		return -EIO;
>  
> +	iomap->flags |= IOMAP_F_BUFFER_HEAD;
> +
>  	if (((flags & (IOMAP_WRITE | IOMAP_DIRECT)) == IOMAP_WRITE) &&
>  			!IS_DAX(inode) && !xfs_get_extsz_hint(ip)) {
>  		/* Reserve delalloc blocks for regular writeback. */
> @@ -1131,7 +1133,7 @@ xfs_file_iomap_begin(
>  	if (error)
>  		return error;
>  
> -	iomap->flags = IOMAP_F_NEW;
> +	iomap->flags |= IOMAP_F_NEW;
>  	trace_xfs_iomap_alloc(ip, offset, length, 0, &imap);
>  
>  out_finish:
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 7300d30ca495..4d3d9d0cd69f 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -30,6 +30,7 @@ struct vm_fault;
>   */
>  #define IOMAP_F_NEW		0x01	/* blocks have been newly allocated */
>  #define IOMAP_F_DIRTY		0x02	/* uncommitted metadata */
> +#define IOMAP_F_BUFFER_HEAD	0x04	/* file system requires buffer heads */
>  
>  /*
>   * Flags that only need to be reported for IOMAP_REPORT requests:
> @@ -92,6 +93,7 @@ ssize_t iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *from,
>  int iomap_readpage(struct page *page, const struct iomap_ops *ops);
>  int iomap_readpages(struct address_space *mapping, struct list_head *pages,
>  		unsigned nr_pages, const struct iomap_ops *ops);
> +int iomap_set_page_dirty(struct page *page);
>  int iomap_file_dirty(struct inode *inode, loff_t pos, loff_t len,
>  		const struct iomap_ops *ops);
>  int iomap_zero_range(struct inode *inode, loff_t pos, loff_t len,
> -- 
> 2.17.0
> 
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