Re: [PATCH 02/13] iomap: Read page from srcmap for IOMAP_COW

[Dave Chinner <david@fromorbit.com>]

On Fri, Aug 02, 2019 at 05:00:37PM -0500, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> In case of a IOMAP_COW, read a page from the srcmap before
> performing a write on the page.
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>  fs/iomap/buffered-io.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index f27756c0b31c..a96cc26eec92 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -581,7 +581,7 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len,
>  
>  static int
>  iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> -		struct page **pagep, struct iomap *iomap)
> +		struct page **pagep, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	const struct iomap_page_ops *page_ops = iomap->page_ops;
>  	pgoff_t index = pos >> PAGE_SHIFT;
> @@ -607,6 +607,8 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>  
>  	if (iomap->type == IOMAP_INLINE)
>  		iomap_read_inline_data(inode, page, iomap);
> +	else if (iomap->type == IOMAP_COW)
> +		status = __iomap_write_begin(inode, pos, len, page, srcmap);
>  	else if (iomap->flags & IOMAP_F_BUFFER_HEAD)
>  		status = __block_write_begin_int(page, pos, len, NULL, iomap);
>  	else

This looks busted w.r.t. IOMAP_F_BUFFER_HEAD.  i.e. What's to stop
someone returning iomap->type == IOMAP_COW, iomap->flags &
IOMAP_F_BUFFER_HEAD?

In which case we can't call __iomap_write_begin(), right?

I'm with Darrick on CONFIG_IOMAP_DEBUG here - we need to start
locking down invalid behaviour and invalid combinations with asserts
that tell developers they've broken something.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com