ext4: fix data corruption for mmap writes

static int mpage_submit_page(struct mpage_da_data *mpd, struct page *page)

{

	int len;

	loff_t size = i_size_read(mpd->inode);

	loff_t size;

	int err;

	BUG_ON(page->index != mpd->first_page);

	clear_page_dirty_for_io(page);

	/*

	 * We have to be very careful here!  Nothing protects writeback path

	 * against i_size changes and the page can be writeably mapped into

	 * page tables. So an application can be growing i_size and writing

	 * data through mmap while writeback runs. clear_page_dirty_for_io()

	 * write-protects our page in page tables and the page cannot get

	 * written to again until we release page lock. So only after

	 * clear_page_dirty_for_io() we are safe to sample i_size for

	 * ext4_bio_write_page() to zero-out tail of the written page. We rely

	 * on the barrier provided by TestClearPageDirty in

	 * clear_page_dirty_for_io() to make sure i_size is really sampled only

	 * after page tables are updated.

	 */

	size = i_size_read(mpd->inode);

	if (page->index == size >> PAGE_SHIFT)

		len = size & ~PAGE_MASK;

	else

		len = PAGE_SIZE;

	clear_page_dirty_for_io(page);

	err = ext4_bio_write_page(&mpd->io_submit, page, len, mpd->wbc, false);

	if (!err)

		mpd->wbc->nr_to_write--;