[PATCH 0/3] block: Keep auto_backing_file post-migration

[Hanna Reitz]

Hi,

https://gitlab.com/qemu-project/qemu/-/issues/1117 reports the following
issue:

Say you have a VM with a backing chain of images where the image
metadata contains json:{} backing file strings, which however will be
resolved to simple plain filenames when opened[1].

So when these images are opened, bs->auto_backing_file is first read
directly from the image header, and will thus contain a json:{}
filename.  The backing image is opened based off of this filename, and
bdrv_refresh_filename() simplfies the filename as shown[1].  We then
update bs->auto_backing_file from bs->backing->bs->filename, so both are
equal.

It is quite important that both are equal, because
bdrv_backing_overridden() checks whether the backing file has been
changed from the default by comparing bs->auto_backing_file to
bs->backing->bs->filename.

Because we did set bs->auto_backing_file from bs->backing->bs->filename,
both are equal, the backing file is not considered overridden, and
bdrv_refresh_filename(bs) will not consider it necessary to generate a
json:{} filename for the overlay.

Then the VM is migrated.

The destination side invokes bdrv_invalidate_cache(), which by qcow2 and
qed is implemented by closing the image and opening it.  This re-reads
the backing file string from disk, resetting bs->auto_backing_file.
Now, it will contains the json:{} filename again and thus differ from
bs->backing->bs->filename.

Consequentially, a subsequent bdrv_refresh_filename(bs) will find that
the overlay’s backing file has been overridden and generate a json:{}
filename, which isn’t great.

This series fixes that by having qcow2’s and qed’s image-open operations
not overwrite bs->auto_backing_file unless something has changed since
the last time we read the backing filename from the metadata.


Now, generating a json:{} filename can be a nuisance but shouldn’t be a
real problem.  The actual problem reported in 1117 comes later, namely
when creating a snapshot overlay post-migration.  This overlay image
will have a json:{} backing filename in its image metadata, which
contains a 'backing' key[2].

'qemu-img info' uses the BDRV_O_NO_BACKING flag to open images, which
conflicts with those backing options: With that flag, nobody processes
those options, and that’s an error.  Therefore, you can’t run 'qemu-img
info --backing-chain' on that overlay image.

That part of the issue is not fixed in this series, however.  I’ll send
a separate RFC series for it, because I’m honstly not quite certain how
it should be fixed.


[1] Example:
        json:{"driver": "qcow2",
              "file": {"driver": "file", "filename": "img.qcow2"}}
    Will generally be “resolved” by bdrv_refresh_filename() to
        "img.qcow2"

[2] That it contains a 'backing' key is only natural, because the reason
    why bdrv_refresh_filename() decided to generate a json:{} filename
    for the image is because it considered the backing file overridden.
    Hence it must put the actual backing file options into a 'backing'
    object in the json:{} filename.


Hanna Reitz (3):
  block/qcow2: Keep auto_backing_file if possible
  block/qed: Keep auto_backing_file if possible
  iotests/backing-file-invalidation: Add new test

 block/qcow2.c                                 |  21 ++-
 block/qed.c                                   |  15 +-
 .../tests/backing-file-invalidation           | 152 ++++++++++++++++++
 .../tests/backing-file-invalidation.out       |   5 +
 4 files changed, 184 insertions(+), 9 deletions(-)
 create mode 100755 tests/qemu-iotests/tests/backing-file-invalidation
 create mode 100644 tests/qemu-iotests/tests/backing-file-invalidation.out

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2.36.1