1 files changed, 33 insertions, 8 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 9e9779967dd..f7376ce74fe 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -2117,14 +2117,39 @@ permissions.  */)
     newsize = st.st_size;
   else
     {
-      char buf[MAX_ALLOCA];
-      ptrdiff_t n;
-      for (newsize = 0; 0 < (n = emacs_read_quit (ifd, buf, sizeof buf));
-           newsize += n)
-        if (emacs_write_quit (ofd, buf, n) != n)
-          report_file_error ("Write error", newname);
-      if (n < 0)
-        report_file_error ("Read error", file);
+      off_t insize = st.st_size;
+      ssize_t copied;
+
+      for (newsize = 0; newsize < insize; newsize += copied)
+        {
+          /* Copy at most COPY_MAX bytes at a time; this is min
+             (PTRDIFF_MAX, SIZE_MAX) truncated to a value that is
+             surely aligned well.  */
+          ptrdiff_t copy_max = min (PTRDIFF_MAX, SIZE_MAX) >> 30 << 30;
+          off_t intail = insize - newsize;
+          ptrdiff_t len = min (intail, copy_max);
+          copied = copy_file_range (ifd, NULL, ofd, NULL, len, 0);
+          if (copied <= 0)
+            break;
+          maybe_quit ();
+        }
+
+      /* Fall back on read+write if copy_file_range failed, or if the
+         input is empty and so could be a /proc file.  read+write will
+         either succeed, or report an error more precisely than
+         copy_file_range would.  */
+      if (newsize != insize || insize == 0)
+        {
+          char buf[MAX_ALLOCA];
+          for (; (copied = emacs_read_quit (ifd, buf, sizeof buf));
+               newsize += copied)
+            {
+              if (copied < 0)
+                report_file_error ("Read error", file);
+              if (emacs_write_quit (ofd, buf, copied) != copied)
+                report_file_error ("Write error", newname);
+            }
+        }
     }
 
   /* Truncate any existing output file after writing the data.  This