RE: [PATCH v10 02/10] fs/ntfs3: Add initialization of super block

[Konstantin Komarov]

From: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Sent: Friday, October 23, 2020 9:25 PM
> To: Konstantin Komarov <almaz.alexandrovich@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx; viro@xxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; pali@xxxxxxxxxx; dsterba@xxxxxxx;
> aaptel@xxxxxxxx; rdunlap@xxxxxxxxxxxxx; joe@xxxxxxxxxxx; mark@xxxxxxxxxxxxx; nborisov@xxxxxxxx; linux-ntfs-
> dev@xxxxxxxxxxxxxxxxxxxxx; anton@xxxxxxxxxx
> Subject: Re: [PATCH v10 02/10] fs/ntfs3: Add initialization of super block
> 
> On Fri, Oct 23, 2020 at 06:44:23PM +0300, Konstantin Komarov wrote:
> > +
> > +/*ntfs_readpage*/
> > +/*ntfs_readpages*/
> > +/*ntfs_writepage*/
> > +/*ntfs_writepages*/
> > +/*ntfs_block_truncate_page*/
> 
> What are these for?
> 
> > +int ntfs_readpage(struct file *file, struct page *page)
> > +{
> > +	int err;
> > +	struct address_space *mapping = page->mapping;
> > +	struct inode *inode = mapping->host;
> > +	struct ntfs_inode *ni = ntfs_i(inode);
> > +	u64 vbo = (u64)page->index << PAGE_SHIFT;
> > +	u64 valid;
> > +	struct ATTRIB *attr;
> > +	const char *data;
> > +	u32 data_size;
> > +
> [...]
> > +
> > +	if (is_compressed(ni)) {
> > +		if (PageUptodate(page)) {
> > +			unlock_page(page);
> > +			return 0;
> > +		}
> 
> You can skip this -- the readpage op won't be called for pages which
> are Uptodate.
> 
> > +	/* normal + sparse files */
> > +	err = mpage_readpage(page, ntfs_get_block);
> > +	if (err)
> > +		goto out;
> 
> It would be nice to use iomap instead of mpage, but that's a big ask.
> 
> > +	valid = ni->i_valid;
> > +	if (vbo < valid && valid < vbo + PAGE_SIZE) {
> > +		if (PageLocked(page))
> > +			wait_on_page_bit(page, PG_locked);
> > +		if (PageError(page)) {
> > +			ntfs_inode_warn(inode, "file garbage at 0x%llx", valid);
> > +			goto out;
> > +		}
> > +		zero_user_segment(page, valid & (PAGE_SIZE - 1), PAGE_SIZE);
> 
> Nono, you can't zero data after the page has been unlocked.  You can
> handle this case in ntfs_get_block().  If the block is entirely beyond
> i_size, returning a hole will cause mpage_readpage() to zero it.  If it
> straddles i_size, you can either ensure that the on-media block contains
> zeroes after the EOF, or if you can't depend on that, you can read it
> in synchronously in your get_block() and then zero the tail and set the
> buffer Uptodate.  Not the most appetising solution, but what you have here
> is racy with the user writing to it after reading.

Hello Matthew! Thanks a lot for this feedback. Fixed in v11, please check out.

Cheers!