On 2021/4/17 上午1:40, David Sterba wrote:
On Fri, Mar 26, 2021 at 08:50:46PM +0800, Qu Wenruo wrote:
If restoring dumpped image into a new file, under most cases kernel will
reject it:

  # mkfs.btrfs -f /dev/test/test
  # btrfs-image /dev/test/test /tmp/dump
  # btrfs-image -r /tmp/dump ~/test.img
  # mount ~/test.img /mnt/btrfs
  mount: /mnt/btrfs: wrong fs type, bad option, bad superblock on /dev/loop0, 
missing codepage or helper program, or other error.
  # dmesg -t | tail -n 7
  loop0: detected capacity change from 10592 to 0
  BTRFS info (device loop0): disk space caching is enabled
  BTRFS info (device loop0): has skinny extents
  BTRFS info (device loop0): flagging fs with big metadata feature
  BTRFS error (device loop0): device total_bytes should be at most 5423104 but 
found 10737418240
  BTRFS error (device loop0): failed to read chunk tree: -22
  BTRFS error (device loop0): open_ctree failed

When btrfs-image restores an image into a file, and the source image
contains only single device, then we don't need to modify the
chunk/device tree, as we can reuse the existing chunk/dev tree without
any problem.

This also means, for such restore, we also won't do any target file
enlarge. This behavior itself is fine, as at that time, kernel won't
check if the device is smaller than the device size recorded in device

But later kernel commit 3a160a933111 ("btrfs: drop never met disk total
bytes check in verify_one_dev_extent") introduces new check on device
size at mount time, rejecting any loop file which is smaller than the
original device size.

Do extra file enlarge for single device restore.

Reported-by: Nikolay Borisov <nbori...@suse.com>
Signed-off-by: Qu Wenruo <w...@suse.com>
  image/main.c | 43 +++++++++++++++++++++++++++++++++++++++++++
  1 file changed, 43 insertions(+)

diff --git a/image/main.c b/image/main.c
index 24393188e5e3..9933f69d0fdb 100644
--- a/image/main.c
+++ b/image/main.c
@@ -2706,6 +2706,49 @@ static int restore_metadump(const char *input, FILE 
*out, int old_restore,
                if (ret)
                        goto out;
+       } else {
+               struct btrfs_root *root;
+               struct stat st;
+               u64 dev_size;
+               if (!info) {
+                       root = open_ctree_fd(fileno(out), target, 0, 0);
+                       if (!root) {
+                               error("open ctree failed in %s", target);
+                               ret = -EIO;
+                               goto out;
+                       }
+                       info = root->fs_info;
+                       dev_size = btrfs_stack_device_total_bytes(
+                                       &info->super_copy->dev_item);
+                       close_ctree(root);
+                       info = NULL;
+               } else {
+                       dev_size = btrfs_stack_device_total_bytes(
+                                       &info->super_copy->dev_item);
+               }
+               /*
+                * We don't need extra tree modification, but if the output is
+                * a file, we need to enlarge the output file so that
+                * newer kernel won't report error.
+                */
+               ret = fstat(fileno(out), &st);
+               if (ret < 0) {
+                       error("failed to stat result image: %m");
+                       ret = -errno;
+                       goto out;
+               }
+               if (S_ISREG(st.st_mode)) {
+                       ret = ftruncate64(fileno(out), dev_size);

This truncates the file unconditionally, so if the file is larger than
required, I don't think it's necessary to do it.

Indeed, I'll update the patchset to do conditional truncation.


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