Update comment for the copy_file_range operation (#497)

copy_file_range was first implemented with copy-based emulation in
glibc 2.27, but the emulation was subsequently removed again because
correct emulation depends on why the application attempted to make a
copy.  Therefore, file systems cannot rely on low-level userspace
performing emulation.

diff --git a/include/fuse.h b/include/fuse.h
index 3df61b1559a905e02360c74a3f40026f9f86789d..a92d5ee8d42a1bbdf2dc42fe74d82e18c7ffe38e 100644 (file)
--- a/include/fuse.h
+++ b/include/fuse.h
@@ -772,9 +772,10 @@ struct fuse_operations {
         * additional cost of transferring data through the FUSE kernel module
         * to user space (glibc) and then back into the FUSE filesystem again.
         *
-        * In case this method is not implemented, glibc falls back to reading
-        * data from the source and writing to the destination. Effectively
-        * doing an inefficient copy of the data.
+        * In case this method is not implemented, applications are expected to
+        * fall back to a regular file copy.   (Some glibc versions did this
+        * emulation automatically, but the emulation has been removed from all
+        * glibc release branches.)
         */
        ssize_t (*copy_file_range) (const char *path_in,
                                    struct fuse_file_info *fi_in,