Re: [PATCH] copy.c: use `sendfile()` for in-kernel file copying on Linux

On 2/15/26 12:43 AM, Phillip Wood wrote:

On 13/02/2026 12:46, George Hu wrote:

quoted

The `sendfile()` system call copies data between one file descriptor
and another within the kernel, which is more efficient than the
combination of `read()` and `write()`.

Does git copy any files big enough that this makes a noticeable 
difference?

quoted

  int copy_fd(int ifd, int ofd)
  {
+#ifdef __linux__

Our normal practice when a function has platform specific 
implementations is to host those implementations under compat/<platform>
(see the implementations of trace2_collect_process_information() for 
an example)

The Linux implementation of `trace2_collect_process_information()` 
resides in compat/linux with a stub version in compat/stub. After moving 
the Linux-specifc `copy_fd()` implementation into compat/linux, where 
should the generic implementation be placed?

quoted

+    struct stat ifd_st;
+    size_t ifd_len;
+    ssize_t ret = 0;
+
+    fstat(ifd, &ifd_st);

What happens if fstat() fails?

quoted

+    ifd_len = ifd_st.st_size;
+
+    while (ifd_len && (ret = sendfile(ofd, ifd, NULL, ifd_len)) > 0)
+        ifd_len -= (size_t)ret;

This does not propagate errors to the caller, if sendfile() fails the 
function returns 0. write_in_full() handles non-blocking writes, we 
should do the same here if we see EAGAIN. The man page lists various 
restrictions on the file descriptors passed to sendfile() - I'm not 
sure that they affect the uses of copy_file() in git but to be safe we 
should fall back to the read()/write() loop if we see EINVAL.

According to the manual, `sendfile()` returns -1 on failure; a return 
value of 0 indicates EOF.

There are error cases besides EAGAIN and EINVAL. Maybe we should fall 
back to the read() / write() loop for errors other than EAGAIN?

Sincerely,
George

Thanks

Phillip

quoted

+#else
      while (1) {
          char buffer[8192];
          ssize_t len = xread(ifd, buffer, sizeof(buffer));

@@ -19,6 +34,8 @@ int copy_fd(int ifd, int ofd)

          if (write_in_full(ofd, buffer, len) < 0)
              return COPY_WRITE_ERROR;
      }
+#endif
+
      return 0;
  }