Re: [PATCH] fs: don't allocate blocks beyond EOF from __mpage_writepage

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Jan 03, 2023 at 11:44:30AM +0100, Jan Kara wrote:
> When __mpage_writepage() is called for a page beyond EOF, it will go and
> allocate all blocks underlying the page. This is not only unnecessary
> but this way blocks can get leaked (e.g. if a page beyond EOF is marked
> dirty but in the end write fails and i_size is not extended).
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/mpage.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/mpage.c b/fs/mpage.c
> index 0f8ae954a579..9f040c1d5912 100644
> --- a/fs/mpage.c
> +++ b/fs/mpage.c
> @@ -524,6 +524,12 @@ static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
>  	 */
>  	BUG_ON(!PageUptodate(page));
>  	block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
> +	/*
> +	 * Whole page beyond EOF? Skip allocating blocks to avoid leaking
> +	 * space.
> +	 */
> +	if (block_in_file >= (i_size + (1 << blkbits) - 1) >> blkbits)
> +		goto page_is_mapped;
>  	last_block = (i_size - 1) >> blkbits;

Why not simply

	if (block_in_file > last_block)
		goto page_is_mapped;

after last_block has been calculated?