question about copy_file_range() between btrfs filesystem.

[Wang Yugui <wangyugui@e16-tech.com>]

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Hi,

question about copy_file_range() between btrfs filesystem.

test progam:
	attachment file(copy directly from 'man copy_file_range')

test result:
kernel: 6.1.4
1)in/out files in single btrfs subvol: OK

2)in/out files in single btrfs filesystem, but different subvols: OK

3)in/out files in different btrfs filesystems: ERROR Invalid cross-device link
   but as a compare, in/out files in different nfs filesystems(in same server): OK.
Question: 
Should we support copy_file_range() between btrfs filesystem?

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2023/01/11


[-- Attachment #2: copy_file_range.c --]
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#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <unistd.h>

/* On versions of glibc before 2.27, we must invoke copy_file_range()
       using syscall(2) */
static loff_t
copy_file_range(int fd_in, loff_t *off_in, int fd_out,
                     loff_t *off_out, size_t len, unsigned int flags)
{
       return syscall(__NR_copy_file_range, fd_in, off_in, fd_out,
                     off_out, len, flags);
}

int
main(int argc, char **argv)
{
       int fd_in, fd_out;
       struct stat stat;
       loff_t len, ret;

       if (argc != 3) {
              fprintf(stderr, "Usage: %s <source> <destination>\n", argv[0]);
              exit(EXIT_FAILURE);
       }

       fd_in = open(argv[1], O_RDONLY);
       if (fd_in == -1) {
              perror("open (argv[1])");
              exit(EXIT_FAILURE);
       }

       if (fstat(fd_in, &stat) == -1) {
              perror("fstat");
              exit(EXIT_FAILURE);
       }

       len = stat.st_size;

       fd_out = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0644);
       if (fd_out == -1) {
              perror("open (argv[2])");
              exit(EXIT_FAILURE);
       }

       do {
              ret = copy_file_range(fd_in, NULL, fd_out, NULL, len, 0);
              if (ret == -1) {
              perror("copy_file_range");
              exit(EXIT_FAILURE);
              }

              len -= ret;
       } while (len > 0 && ret > 0);

       close(fd_in);
       close(fd_out);
       exit(EXIT_SUCCESS);
}