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

[Max Reitz]

On 27.07.20 14:27, Andrey Shinkevich wrote:
> 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
>> 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).)
>>
>> 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 <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?

I don’t think so.  The format we return in info->backing_filename_format
should be the format of the file returned in info->backing_filename.
The latter is bs->backing_file, which (as of this patch), is the backing
file as reported by the image header.  Therefore, if the backing file
was overridden, we cannot assume that bs->backing->bs refers to the same
file as bs->backing_file, and so we cannot assume
bs->backing->bs->drv->format_name to be info->backing_filename’s format.

Max

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