Re: [PATCH v5 2/3] iomap: Add a page_prepare callback

[Christoph Hellwig <hch@lst.de>]

On Fri, Apr 26, 2019 at 03:11:26PM +0200, Andreas Gruenbacher wrote:
> Move the page_done callback into a separate iomap_page_ops structure and
> add a page_prepare calback to be called before the next page is written
> to.  In gfs2, we'll want to start a transaction in page_prepare and end
> it in page_done; other filesystems that implement data journaling will
> require the same kind of mechanism.
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> ---
>  fs/gfs2/bmap.c        | 22 +++++++++++++++++-----
>  fs/iomap.c            | 22 ++++++++++++++++++----
>  include/linux/iomap.h | 18 +++++++++++++-----
>  3 files changed, 48 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
> index 5da4ca9041c0..6b980703bae7 100644
> --- a/fs/gfs2/bmap.c
> +++ b/fs/gfs2/bmap.c
> @@ -991,15 +991,27 @@ static void gfs2_write_unlock(struct inode *inode)
>  	gfs2_glock_dq_uninit(&ip->i_gh);
>  }
>  
> -static void gfs2_iomap_journaled_page_done(struct inode *inode, loff_t pos,
> -				unsigned copied, struct page *page,
> -				struct iomap *iomap)
> +static int gfs2_iomap_page_prepare(struct inode *inode, loff_t pos,
> +				   unsigned len, struct iomap *iomap)
> +{
> +	return 0;
> +}
> +
> +static void gfs2_iomap_page_done(struct inode *inode, loff_t pos,
> +				 unsigned copied, struct page *page,
> +				 struct iomap *iomap)
>  {
>  	struct gfs2_inode *ip = GFS2_I(inode);
>  
> -	gfs2_page_add_databufs(ip, page, offset_in_page(pos), copied);
> +	if (page)
> +		gfs2_page_add_databufs(ip, page, offset_in_page(pos), copied);
>  }
>  
> +static const struct iomap_page_ops gfs2_iomap_page_ops = {
> +	.page_prepare = gfs2_iomap_page_prepare,
> +	.page_done = gfs2_iomap_page_done,
> +};
> +
>  static int gfs2_iomap_begin_write(struct inode *inode, loff_t pos,
>  				  loff_t length, unsigned flags,
>  				  struct iomap *iomap,
> @@ -1077,7 +1089,7 @@ static int gfs2_iomap_begin_write(struct inode *inode, loff_t pos,
>  		}
>  	}
>  	if (!gfs2_is_stuffed(ip) && gfs2_is_jdata(ip))
> -		iomap->page_done = gfs2_iomap_journaled_page_done;
> +		iomap->page_ops = &gfs2_iomap_page_ops;
>  	return 0;
>  
>  out_trans_end:
> diff --git a/fs/iomap.c b/fs/iomap.c
> index 3e4652dac9d9..ba2d44b33ed1 100644
> --- a/fs/iomap.c
> +++ b/fs/iomap.c
> @@ -665,6 +665,7 @@ static int
>  iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>  		struct page **pagep, struct iomap *iomap)
>  {
> +	const struct iomap_page_ops *page_ops = iomap->page_ops;
>  	pgoff_t index = pos >> PAGE_SHIFT;
>  	struct page *page;
>  	int status = 0;
> @@ -674,9 +675,17 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>  	if (fatal_signal_pending(current))
>  		return -EINTR;
>  
> +	if (page_ops) {
> +		status = page_ops->page_prepare(inode, pos, len, iomap);
> +		if (status)
> +			return status;
> +	}
> +
>  	page = grab_cache_page_write_begin(inode->i_mapping, index, flags);
> -	if (!page)
> -		return -ENOMEM;
> +	if (!page) {
> +		status = -ENOMEM;
> +		goto no_page;
> +	}
>  
>  	if (iomap->type == IOMAP_INLINE)
>  		iomap_read_inline_data(inode, page, iomap);
> @@ -684,12 +693,16 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>  		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);
>  		page = NULL;
>  
>  		iomap_write_failed(inode, pos, len);
> +no_page:
> +		if (page_ops)
> +			page_ops->page_done(inode, pos, 0, NULL, iomap);
>  	}
>  
>  	*pagep = page;

I think we need to clean this area up a bit, this is becoming to
confusing.

Something like:

	if (unlikely(status))
		goto out_unlock;

	*pagep = page;
	return 0;

out_unlock:
	unlock_page(page);
	put_page(page);
	iomap_write_failed(inode, pos, len);
out_no_page:
	if (page_ops)
		page_ops->page_done(inode, pos, 0, NULL, iomap);
	*pagep = NULL;
	return status;


> +	if (page_ops)
> +		page_ops->page_done(inode, pos, copied, page, iomap);

Do we always require both pages ops to be set?  Wouldn't it be better
to check for the method presence as well?

> +/*
> + * Called before / after processing a page in the mapping returned in this
> + * iomap.  At least for now, this is only supported in the buffered write path.
> + * When page_prepare returns 0, page_done is called as well
> + * (possibly with page == NULL).
> + */

Not just for now - I think the concept fundamentally only makes sense
for buffered I/O.  Maybe we need to express that a little more clearly.

Otherwise looks fine to me.