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

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Date:   Fri, 23 Oct 2020 19:25:03 +0100
From:   Matthew Wilcox <willy@...radead.org>
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.