[RFC][PATCH][cp] btrfs, nocow and cp --reflink

* [RFC][PATCH][cp] btrfs, nocow and cp --reflink
@ 2022-05-25 17:05 Goffredo Baroncelli
  2022-05-27 14:00 ` Pádraig Brady
  0 siblings, 1 reply; 3+ messages in thread
From: Goffredo Baroncelli @ 2022-05-25 17:05 UTC (permalink / raw)
  To: linux-btrfs, coreutils; +Cc: Forza, Matthew Warren, Andrei Borzenkov

Hi All,

recently I discovered that BTRFS allow to reflink a file only if the flag FS_NOCOW_FL is the same on both source and destination.
In the end of this email I added a patch to "cp" to set the FS_NOCOW_FL flag according to the source.

Even tough this works, I am wondering if this is the expected/the least surprise behavior by/for any user. This is the reason why this email is tagged as RFC.

Without reflink, the default behavior is that the new file has the FS_NOCOW_FL flag set according to the parent directory; with this patch the flag would be the same as the source.

I am not sure that this is the correct behviour without warning the user of this change.

Another possibility, is to flip the NOCOW flag only if --reflink=always is passed.

Thoughts ?

BR
G.Baroncelli


-----

diff --git a/src/copy.c b/src/copy.c
index b15d91990..41df45cd5 100644
--- a/src/copy.c
+++ b/src/copy.c
@@ -22,6 +22,8 @@
  #include <sys/ioctl.h>
  #include <sys/types.h>
  #include <selinux/selinux.h>
+#include <sys/vfs.h>
+#include <linux/magic.h>
  
  #if HAVE_HURD_H
  # include <hurd.h>
@@ -399,6 +401,32 @@ static inline int
  clone_file (int dest_fd, int src_fd)
  {
  #ifdef FICLONE
+# ifdef __linux__
+  /* BTRFS requires that both source and dest have the same setting
+     about FS_NOCOW_FL */
+  int src_flags, dst_flags, r;
+  struct statfs sbuf;
+
+  r = fstatfs(dest_fd, &sbuf);
+  if (r < 0)
+    return r;
+  if (sbuf.f_type == BTRFS_SUPER_MAGIC || sbuf.f_type == BTRFS_TEST_MAGIC)
+    {
+      r = ioctl(src_fd, FS_IOC_GETFLAGS, &src_flags);
+      if (r < 0)
+        return r;
+      r = ioctl(dest_fd, FS_IOC_GETFLAGS, &dst_flags);
+      if (r < 0)
+        return r;
+      if ((src_flags ^ dst_flags) & FS_NOCOW_FL)
+        {
+          dst_flags ^= FS_NOCOW_FL;
+          r = ioctl(dest_fd, FS_IOC_SETFLAGS, &dst_flags);
+          if (r < 0)
+            return r;
+        }
+    }
+# endif
    return ioctl (dest_fd, FICLONE, src_fd);
  #else
    (void) dest_fd;



-- 
gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it>
Key fingerprint BBF5 1610 0B64 DAC6 5F7D  17B2 0EDA 9B37 8B82 E0B5

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