Re: [PATCH v14 078/138] mm/filemap: Add folio_mkwrite_check_truncate()

[Vlastimil Babka]

On 7/15/21 5:36 AM, Matthew Wilcox (Oracle) wrote:
> This is the folio equivalent of page_mkwrite_check_truncate().
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>  include/linux/pagemap.h | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 412db88b8d0c..18c06c3e42c3 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -1121,6 +1121,34 @@ static inline unsigned long dir_pages(struct inode *inode)
>  			       PAGE_SHIFT;
>  }
>  
> +/**
> + * folio_mkwrite_check_truncate - check if folio was truncated
> + * @folio: the folio to check
> + * @inode: the inode to check the folio against
> + *
> + * Return: the number of bytes in the folio up to EOF,
> + * or -EFAULT if the folio was truncated.
> + */
> +static inline ssize_t folio_mkwrite_check_truncate(struct folio *folio,
> +					      struct inode *inode)
> +{
> +	loff_t size = i_size_read(inode);
> +	pgoff_t index = size >> PAGE_SHIFT;
> +	size_t offset = offset_in_folio(folio, size);
> +
> +	if (!folio->mapping)

The check in the page_ version is
if (page->mapping != inode->i_mapping)

Why is the one above sufficient?

> +		return -EFAULT;
> +
> +	/* folio is wholly inside EOF */
> +	if (folio_next_index(folio) - 1 < index)
> +		return folio_size(folio);
> +	/* folio is wholly past EOF */
> +	if (folio->index > index || !offset)
> +		return -EFAULT;
> +	/* folio is partially inside EOF */
> +	return offset;
> +}
> +
>  /**
>   * page_mkwrite_check_truncate - check if page was truncated
>   * @page: the page to check
>