Re: [PATCH 06/20] iomap: use write_begin to read pages to unshare

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

On Tue, Oct 08, 2019 at 09:15:13AM +0200, Christoph Hellwig wrote:
> Use the existing iomap write_begin code to read the pages unshared
> by iomap_file_unshare.  That avoids the extra ->readpage call and
> extent tree lookup currently done by read_mapping_page.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Looks ok,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  fs/iomap/buffered-io.c | 49 ++++++++++++++----------------------------
>  1 file changed, 16 insertions(+), 33 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d5abd8e5dca7..ac1bbed71a9b 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -548,6 +548,10 @@ iomap_migrate_page(struct address_space *mapping, struct page *newpage,
>  EXPORT_SYMBOL_GPL(iomap_migrate_page);
>  #endif /* CONFIG_MIGRATION */
>  
> +enum {
> +	IOMAP_WRITE_F_UNSHARE		= (1 << 0),
> +};
> +
>  static void
>  iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
>  {
> @@ -577,7 +581,7 @@ iomap_read_page_sync(loff_t block_start, struct page *page, unsigned poff,
>  }
>  
>  static int
> -__iomap_write_begin(struct inode *inode, loff_t pos, unsigned len,
> +__iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
>  		struct page *page, struct iomap *iomap)
>  {
>  	struct iomap_page *iop = iomap_page_create(inode, page);
> @@ -596,11 +600,14 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len,
>  		if (plen == 0)
>  			break;
>  
> -		if ((from <= poff || from >= poff + plen) &&
> +		if (!(flags & IOMAP_WRITE_F_UNSHARE) &&
> +		    (from <= poff || from >= poff + plen) &&
>  		    (to <= poff || to >= poff + plen))
>  			continue;
>  
>  		if (iomap_block_needs_zeroing(inode, iomap, block_start)) {
> +			if (WARN_ON_ONCE(flags & IOMAP_WRITE_F_UNSHARE))
> +				return -EIO;
>  			zero_user_segments(page, poff, from, to, poff + plen);
>  			iomap_set_range_uptodate(page, poff, plen);
>  			continue;
> @@ -646,7 +653,8 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>  	else 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);
> +		status = __iomap_write_begin(inode, pos, len, flags, page,
> +				iomap);
>  
>  	if (unlikely(status))
>  		goto out_unlock;
> @@ -869,22 +877,6 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *iter,
>  }
>  EXPORT_SYMBOL_GPL(iomap_file_buffered_write);
>  
> -static struct page *
> -__iomap_read_page(struct inode *inode, loff_t offset)
> -{
> -	struct address_space *mapping = inode->i_mapping;
> -	struct page *page;
> -
> -	page = read_mapping_page(mapping, offset >> PAGE_SHIFT, NULL);
> -	if (IS_ERR(page))
> -		return page;
> -	if (!PageUptodate(page)) {
> -		put_page(page);
> -		return ERR_PTR(-EIO);
> -	}
> -	return page;
> -}
> -
>  static loff_t
>  iomap_unshare_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  		struct iomap *iomap)
> @@ -900,24 +892,15 @@ iomap_unshare_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  		return length;
>  
>  	do {
> -		struct page *page, *rpage;
> -		unsigned long offset;	/* Offset into pagecache page */
> -		unsigned long bytes;	/* Bytes to write to page */
> -
> -		offset = offset_in_page(pos);
> -		bytes = min_t(loff_t, PAGE_SIZE - offset, length);
> -
> -		rpage = __iomap_read_page(inode, pos);
> -		if (IS_ERR(rpage))
> -			return PTR_ERR(rpage);
> +		unsigned long offset = offset_in_page(pos);
> +		unsigned long bytes = min_t(loff_t, PAGE_SIZE - offset, length);
> +		struct page *page;
>  
> -		status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap);
> -		put_page(rpage);
> +		status = iomap_write_begin(inode, pos, bytes,
> +				IOMAP_WRITE_F_UNSHARE, &page, iomap);
>  		if (unlikely(status))
>  			return status;
>  
> -		WARN_ON_ONCE(!PageUptodate(page));
> -
>  		status = iomap_write_end(inode, pos, bytes, bytes, page, iomap);
>  		if (unlikely(status <= 0)) {
>  			if (WARN_ON_ONCE(status == 0))
> -- 
> 2.20.1
>