fuse: copy_file_range should truncate cache

After the copy operation completes the cache is not up-to-date.  Truncate
all pages in the interval that has successfully been copied.

Truncating completely copied dirty pages is okay, since the data has been
overwritten anyway.  Truncating partially copied dirty pages is not okay;
add a comment for now.

Fixes: 88bc7d50 ("fuse: add support for copy_file_range()")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -3337,6 +3337,24 @@ static ssize_t __fuse_copy_file_range(struct file *file_in, loff_t pos_in,
 	if (err)
 		goto out;
 
+	/*
+	 * Write out dirty pages in the destination file before sending the COPY
+	 * request to userspace.  After the request is completed, truncate off
+	 * pages (including partial ones) from the cache that have been copied,
+	 * since these contain stale data at that point.
+	 *
+	 * This should be mostly correct, but if the COPY writes to partial
+	 * pages (at the start or end) and the parts not covered by the COPY are
+	 * written through a memory map after calling fuse_writeback_range(),
+	 * then these partial page modifications will be lost on truncation.
+	 *
+	 * It is unlikely that someone would rely on such mixed style
+	 * modifications.  Yet this does give less guarantees than if the
+	 * copying was performed with write(2).
+	 *
+	 * To fix this a i_mmap_sem style lock could be used to prevent new
+	 * faults while the copy is ongoing.
+	 */
 	err = fuse_writeback_range(inode_out, pos_out, pos_out + len - 1);
 	if (err)
 		goto out;
@@ -3360,6 +3378,10 @@ static ssize_t __fuse_copy_file_range(struct file *file_in, loff_t pos_in,
 	if (err)
 		goto out;
 
+	truncate_inode_pages_range(inode_out->i_mapping,
+				   ALIGN_DOWN(pos_out, PAGE_SIZE),
+				   ALIGN(pos_out + outarg.size, PAGE_SIZE) - 1);
+
 	if (fc->writeback_cache) {
 		fuse_write_update_size(inode_out, pos_out + outarg.size);
 		file_update_time(file_out);