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

[Jan Kara <jack@suse.cz>]

On Wed 04-01-23 00:02:31, Al Viro wrote:
> 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?

Because if i_size == 0, last_block is (~0 >> blkbits) (which was actually
the case the test hit).

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR