Re: [PATCH v7 41/47] block: Leave BDS.backing_file constant

[Max Reitz <mreitz@redhat.com>]

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On 24.08.20 15:14, Kevin Wolf wrote:
> Am 25.06.2020 um 17:22 hat Max Reitz geschrieben:
>> Parts of the block layer treat BDS.backing_file as if it were whatever
>> the image header says (i.e., if it is a relative path, it is relative to
>> the overlay), other parts treat it like a cache for
>> bs->backing->bs->filename (relative paths are relative to the CWD).
>> Considering bs->backing->bs->filename exists, let us make it mean the
>> former.
>>
>> Among other things, this now allows the user to specify a base when
>> using qemu-img to commit an image file in a directory that is not the
>> CWD (assuming, everything uses relative filenames).
>>
>> Before this patch:
>>
>> $ ./qemu-img create -f qcow2 foo/bot.qcow2 1M
>> $ ./qemu-img create -f qcow2 -b bot.qcow2 foo/mid.qcow2
>> $ ./qemu-img create -f qcow2 -b mid.qcow2 foo/top.qcow2
>> $ ./qemu-img commit -b mid.qcow2 foo/top.qcow2
>> qemu-img: Did not find 'mid.qcow2' in the backing chain of 'foo/top.qcow2'
>> $ ./qemu-img commit -b foo/mid.qcow2 foo/top.qcow2
>> qemu-img: Did not find 'foo/mid.qcow2' in the backing chain of 'foo/top.qcow2'
>> $ ./qemu-img commit -b $PWD/foo/mid.qcow2 foo/top.qcow2
>> qemu-img: Did not find '[...]/foo/mid.qcow2' in the backing chain of 'foo/top.qcow2'
>>
>> After this patch:
>>
>> $ ./qemu-img commit -b mid.qcow2 foo/top.qcow2
>> Image committed.
>> $ ./qemu-img commit -b foo/mid.qcow2 foo/top.qcow2
>> qemu-img: Did not find 'foo/mid.qcow2' in the backing chain of 'foo/top.qcow2'
>> $ ./qemu-img commit -b $PWD/foo/mid.qcow2 foo/top.qcow2
>> Image committed.
>>
>> With this change, bdrv_find_backing_image() must look at whether the
>> user has overridden a BDS's backing file.  If so, it can no longer use
>> bs->backing_file, but must instead compare the given filename against
>> the backing node's filename directly.
>>
>> Note that this changes the QAPI output for a node's backing_file.  We
>> had very inconsistent output there (sometimes what the image header
>> said, sometimes the actual filename of the backing image).  This
>> inconsistent output was effectively useless, so we have to decide one
>> way or the other.  Considering that bs->backing_file usually at runtime
>> contained the path to the image relative to qemu's CWD (or absolute),
>> this patch changes QAPI's backing_file to always report the
>> bs->backing->bs->filename from now on.  If you want to receive the image
>> header information, you have to refer to full-backing-filename.
>>
>> This necessitates a change to iotest 228.  The interesting information
>> it really wanted is the image header, and it can get that now, but it
>> has to use full-backing-filename instead of backing_file.  Because of
>> this patch's changes to bs->backing_file's behavior, we also need some
>> reference output changes.
>>
>> Along with the changes to bs->backing_file, stop updating
>> BDS.backing_format in bdrv_backing_attach() as well.  In order not to
>> change our externally visible behavior (incompatibly), we have to let
>> bdrv_query_image_info() try to get the image format from bs->backing if
>> bs->backing_format is unset.  (The QAPI schema describes
>> backing-filename-format as "the format of the backing file", so it is
>> not necessarily what the image header says, but just the format of the
>> file referenced by backing-filename (if known).)
> 
> Why is it okay to change backing-filename incompatibly, but not
> backing-filename-format?

I hope you’re asking the reverse, i.e. why I don’t change
backing-filename-format, too.  The answer to that is yeah, why not. :)

> I would find it much more consistent if
> ImageInfo reported the value from the header in both fields, and
> BlockDeviceInfo reported the values actually in use.
> 
> The QAPI schema described ImageInfo as "Information about a QEMU image
> file" and runtime state really isn't information about an image file.
> 
> If you want to know the probed image format, you can still look at
> backing-image.format. I don't think this change is much different from
> what you described above for BlockDeviceInfo.backing_file.

Well, OK then.

Max


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