bdev: Do not return EBUSY if bdev discard races with write

blkdev_fallocate() tries to detect whether a discard raced with an
overlapping write by calling invalidate_inode_pages2_range(). However
this check can give both false negatives (when writing using direct IO
or when writeback already writes out the written pagecache range) and
false positives (when write is not actually overlapping but ends in the
same page when blocksize < pagesize). This actually causes issues for
qemu which is getting confused by EBUSY errors.

Fix the problem by removing this conflicting write detection since it is
inherently racy and thus of little use anyway.
Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
CC: "Darrick J. Wong" <darrick.wong@oracle.com>
Link: https://lore.kernel.org/qemu-devel/20201111153913.41840-1-mlevitsk@redhat.comSigned-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/block_dev.c

@@ -1801,13 +1801,11 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
 		return error;
 
 	/*
-	 * Invalidate again; if someone wandered in and dirtied a page,
-	 * the caller will be given -EBUSY.  The third argument is
-	 * inclusive, so the rounding here is safe.
+	 * Invalidate the page cache again; if someone wandered in and dirtied
+	 * a page, we just discard it - userspace has no way of knowing whether
+	 * the write happened before or after discard completing...
 	 */
-	return invalidate_inode_pages2_range(bdev->bd_inode->i_mapping,
-					     start >> PAGE_SHIFT,
-					     end >> PAGE_SHIFT);
+	return truncate_bdev_range(bdev, file->f_mode, start, end);
 }
 
 const struct file_operations def_blk_fops = {