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

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Message-ID: <afb07a383dc747398f65ac541206b562@paragon-software.com>
Date:   Fri, 30 Oct 2020 15:54:19 +0000
From:   Konstantin Komarov <almaz.alexandrovich@...agon-software.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "pali@...nel.org" <pali@...nel.org>,
        "dsterba@...e.cz" <dsterba@...e.cz>,
        "aaptel@...e.com" <aaptel@...e.com>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "joe@...ches.com" <joe@...ches.com>,
        "mark@...mstone.com" <mark@...mstone.com>,
        "nborisov@...e.com" <nborisov@...e.com>,
        "linux-ntfs-dev@...ts.sourceforge.net" 
        <linux-ntfs-dev@...ts.sourceforge.net>,
        "anton@...era.com" <anton@...era.com>
Subject: RE: [PATCH v10 02/10] fs/ntfs3: Add initialization of super block

From: Matthew Wilcox <willy@...radead.org>
Sent: Friday, October 23, 2020 9:25 PM
> To: Konstantin Komarov <almaz.alexandrovich@...agon-software.com>
> Cc: linux-fsdevel@...r.kernel.org; viro@...iv.linux.org.uk; linux-kernel@...r.kernel.org; pali@...nel.org; dsterba@...e.cz;
> aaptel@...e.com; rdunlap@...radead.org; joe@...ches.com; mark@...mstone.com; nborisov@...e.com; linux-ntfs-
> dev@...ts.sourceforge.net; anton@...era.com
> 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!