Re: [PATCH] copy: adjust fiemap handling of sparse files

[Mike Frysinger]

On Wednesday, March 16, 2011 19:55:56 Pádraig Brady wrote:
> On 16/03/11 19:18, Mike Frysinger wrote:
> > well, in the bug report i was working with, we were seeing data loss.  i
> > wonder if it'd be possible to detect the fs/kernel version and error out
> > when versions are found that are known to be broken ?
> 
> That was a different issue.
> 
> It would be nice, but I don't think it would be practical to try and detect
> and work around such file system issues in cp.
> 
> There are always going to be teething problems with a large
> body of new logic, so I think the best approach with file systems
> is to increase trust in the gradually over time.

while this is true, i'd understand if the issue were bugs in coreutils' cp.
they'd get fixed and a new release made, no problem.  but we're talking about
silent errors in the kernel, so anyone running a newer coreutils with a kernel
from two days ago is going hit issues.  if we were looking at basic read/write
functionality, i'd understand not bothering, but we're talking purely about
optimization paths in coreutils' cp.

on the Gentoo side, i guess i'll run with a hack like so:

--- a/src/copy.c
+++ b/src/copy.c
@@ -22,6 +22,7 @@
 #include <sys/ioctl.h>
 #include <sys/types.h>
 #include <selinux/selinux.h>
+#include <sys/utsname.h>
 
 #if HAVE_HURD_H
 # include <hurd.h>
@@ -930,7 +931,32 @@ copy_reg (char const *src_name, char const *dst_name,
              the file is a hole.  */
           if (x->sparse_mode == SPARSE_AUTO && S_ISREG (src_open_sb.st_mode)
               && ST_NBLOCKS (src_open_sb) < src_open_sb.st_size / 
ST_NBLOCKSIZE)
-            make_holes = true;
+            {
+              make_holes = true;
+
+# ifdef __linux__
+              static int safe_kernel = -1;
+
+              if (safe_kernel == -1)
+                {
+                  struct utsname name;
+
+                  safe_kernel = 1;
+
+                  if (uname (&name) != -1 && strncmp (name.release, "2.6.", 4) 
== 0)
+                    {
+                      int kver = atoi (name.release + 4);
+
+                      /* ext4 & btrfs are known to be broken */
+                      if (kver < 38)
+                        safe_kernel = 0;
+                    }
+                }
+
+              if (safe_kernel == 0)
+                make_holes = false;
+# endif
+            }
 #endif
         }
 
-mike

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