Re: [Qemu-devel] [PATCH v6 37/42] block: Leave BDS.backing_file constant
09.08.2019 19:14, Max Reitz wrote:
> 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'
nothing works
>
> 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.
something works.. However it seems that not working one is actually most
probable
to be called by user. Anyway something is better than nothing.
>
> 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. This necessitates
> a change to the reference output of iotest 191.
>
> iotest 245 changes in behavior: With the backing node no longer
> overriding the parent node's backing_file string, you can now omit the
> @backing option when reopening a node with neither a default nor a
> current backing file even if it used to have a backing node at some
> point.
>
> Signed-off-by: Max Reitz
> ---
> include/block/block_int.h | 19 ++-
> block.c| 35 ---
> block/qapi.c | 7 ---
> tests/qemu-iotests/191.out | 1 -
> tests/qemu-iotests/228 | 6 +++---
> tests/qemu-iotests/228.out | 6 +++---
> tests/qemu-iotests/245 | 4 +++-
> 7 files changed, 55 insertions(+), 23 deletions(-)
>
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 42ee2fcf7f..993bafc090 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -784,11 +784,20 @@ struct BlockDriverState {
> bool walking_aio_notifiers; /* to make removal during iteration safe */
>
> char filename[PATH_MAX];
> -char backing_file[PATH_MAX]; /* if non zero, the image is a diff of
> -this file image */
> -/* The backing filename indicated by the image header; if we ever
> - * open this file, then this is replaced by the resulting BDS's
> - * filename (i.e. after a bdrv_refresh_filename() run). */
> +/*
> + * If not empty, this image is a diff in relation to backing_file.
> + * Note that this is the name given in the image header
Is it synced when image header is updated? If yes, it's not constant, if not
it's just wrong.
> and
> + * therefore may or may not be equal to .backing->bs->filename.
> + * If this field contains a relative path, it is to be resolved
> + * relatively to the overlay's location.
> + */
> +char backing_file[PATH_MAX];
> +/*
> + * The backing filename
[Qemu-devel] [PATCH v6 37/42] block: Leave BDS.backing_file constant
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. This necessitates
a change to the reference output of iotest 191.
iotest 245 changes in behavior: With the backing node no longer
overriding the parent node's backing_file string, you can now omit the
@backing option when reopening a node with neither a default nor a
current backing file even if it used to have a backing node at some
point.
Signed-off-by: Max Reitz
---
include/block/block_int.h | 19 ++-
block.c| 35 ---
block/qapi.c | 7 ---
tests/qemu-iotests/191.out | 1 -
tests/qemu-iotests/228 | 6 +++---
tests/qemu-iotests/228.out | 6 +++---
tests/qemu-iotests/245 | 4 +++-
7 files changed, 55 insertions(+), 23 deletions(-)
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 42ee2fcf7f..993bafc090 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -784,11 +784,20 @@ struct BlockDriverState {
bool walking_aio_notifiers; /* to make removal during iteration safe */
char filename[PATH_MAX];
-char backing_file[PATH_MAX]; /* if non zero, the image is a diff of
-this file image */
-/* The backing filename indicated by the image header; if we ever
- * open this file, then this is replaced by the resulting BDS's
- * filename (i.e. after a bdrv_refresh_filename() run). */
+/*
+ * If not empty, this image is a diff in relation to backing_file.
+ * Note that this is the name given in the image header and
+ * therefore may or may not be equal to .backing->bs->filename.
+ * If this field contains a relative path, it is to be resolved
+ * relatively to the overlay's location.
+ */
+char backing_file[PATH_MAX];
+/*
+ * The backing filename indicated by the image header. Contrary
+ * to backing_file, if we ever open this file, auto_backing_file
+ * is replaced by the resulting BDS's filename (i.e. after a
+ * bdrv_refresh_filename() run).
+ */
char auto_backing_file[PATH_MAX];
char backing_format[16]; /* if non-zero and backing_file exists */
diff --git a/block.c b/block.c
index 4858d3e718..88533fa0d3 100644
--- a/block.c
+++ b/block.c
@@ -78,6 +78,8 @@ static BlockDriverState *bdrv_open_inherit(const char
