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

From: Andrey Shinkevich
Subject: Re: [PATCH v7 41/47] block: Leave BDS.backing_file constant
Date: Mon, 27 Jul 2020 15:27:47 +0300
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 25.06.2020 18:22, 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

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 

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).)

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

Signed-off-by: Max Reitz <mreitz@redhat.com>
  include/block/block_int.h  | 21 ++++++++++++++++-----
  block.c                    | 35 +++++++++++++++++++++++++++--------
  block/qapi.c               | 17 +++++++++++++----
  tests/qemu-iotests/228     |  6 +++---
  tests/qemu-iotests/228.out |  6 +++---
  tests/qemu-iotests/245     |  4 +++-
  6 files changed, 65 insertions(+), 24 deletions(-)

diff --git a/block/qapi.c b/block/qapi.c
index 2628323b63..5da6d7e6e0 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -47,7 +47,7 @@ BlockDeviceInfo *bdrv_block_device_info(BlockBackend *blk,
                                          Error **errp)
      ImageInfo **p_image_info;
-    BlockDriverState *bs0;
+    BlockDriverState *bs0, *backing;
      BlockDeviceInfo *info;
if (!bs->drv) {
@@ -76,9 +76,10 @@ BlockDeviceInfo *bdrv_block_device_info(BlockBackend *blk,
          info->node_name = g_strdup(bs->node_name);
- if (bs->backing_file[0]) {
+    backing = bdrv_cow_bs(bs);
+    if (backing) {
          info->has_backing_file = true;
-        info->backing_file = g_strdup(bs->backing_file);
+        info->backing_file = g_strdup(backing->filename);
if (!QLIST_EMPTY(&bs->dirty_bitmaps)) {
@@ -314,6 +315,8 @@ void bdrv_query_image_info(BlockDriverState *bs,
      backing_filename = bs->backing_file;
      if (backing_filename[0] != '\0') {
          char *backing_filename2;
+        const char *backing_format = NULL;
          info->backing_filename = g_strdup(backing_filename);
          info->has_backing_filename = true;
          backing_filename2 = bdrv_get_full_backing_filename(bs, NULL);
@@ -326,7 +329,13 @@ void bdrv_query_image_info(BlockDriverState *bs,
if (bs->backing_format[0]) {
-            info->backing_filename_format = g_strdup(bs->backing_format);
+            backing_format = bs->backing_format;
+        } else if (bs->backing && bs->backing->bs->drv &&
+                   !bdrv_backing_overridden(bs)) {
+            backing_format = bs->backing->bs->drv->format_name;
+        }

In case bdrv_backing_overridden() returns true , should we invoke bdrv_refresh_filename() and assign the format_name then?


+        if (backing_format) {
+            info->backing_filename_format = g_strdup(backing_format);
              info->has_backing_filename_format = true;


Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>

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