Re: [PATCH v2 2/3] afs: Fix afs_write_end() to handle short writes

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jun 17, 2021 at 09:24:16AM +0100, David Howells wrote:
> Fix afs_write_end() to correctly handle a short copy into the intended
> write region of the page.  Two things are necessary:
> 
>  (1) If the page is not up to date, then we should just return 0
>      (ie. indicating a zero-length copy).  The loop in
>      generic_perform_write() will go around again, possibly breaking up the
>      iterator into discrete chunks.
> 
>      This is analogous to commit b9de313cf05fe08fa59efaf19756ec5283af672a
>      for ceph.
> 
>  (2) The page should not have been set uptodate if it wasn't completely set
>      up by netfs_write_begin() (this will be fixed in the next patch), so
>      we need to set uptodate here in such a case.
> 
> Also remove the assertion that was checking that the page was set uptodate
> since it's now set uptodate if it wasn't already a few lines above.  The
> assertion was from when uptodate was set elsewhere.

Thanks for adding that explanation.

> +++ b/fs/afs/write.c
> @@ -119,6 +119,16 @@ int afs_write_end(struct file *file, struct address_space *mapping,
>  	_enter("{%llx:%llu},{%lx}",
>  	       vnode->fid.vid, vnode->fid.vnode, page->index);
>  
> +	len = min_t(size_t, len, thp_size(page) - from);

This line isn't necessary yet, right?

> +	if (!PageUptodate(page)) {
> +		if (copied < len) {
> +			copied = 0;
> +			goto out;
> +		}
> +
> +		SetPageUptodate(page);
> +	}
> +
>  	if (copied == 0)
>  		goto out;
>  
> @@ -133,8 +143,6 @@ int afs_write_end(struct file *file, struct address_space *mapping,
>  		write_sequnlock(&vnode->cb_lock);
>  	}
>  
> -	ASSERT(PageUptodate(page));
> -
>  	if (PagePrivate(page)) {
>  		priv = page_private(page);
>  		f = afs_page_dirty_from(page, priv);

The rest of this looks good.