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From: Max Reitz <mreitz@redhat.com>
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Date: Wed, 29 Aug 2018 11:44:52 +0200
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Subject: Re: [Qemu-devel] [PATCH for-3.1 v10 05/31] block: Respect backing
 bs in bdrv_refresh_filename
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To: Alberto Garcia <berto@igalia.com>, qemu-block@nongnu.org
Cc: qemu-devel@nongnu.org, Kevin Wolf <kwolf@redhat.com>, Eric Blake <eblake@redhat.com>

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From: Max Reitz <mreitz@redhat.com>
To: Alberto Garcia <berto@igalia.com>, qemu-block@nongnu.org
Cc: qemu-devel@nongnu.org, Kevin Wolf <kwolf@redhat.com>,
 Eric Blake <eblake@redhat.com>
Message-ID: <ca1fb933-8a5b-3055-0460-ab1d3859ebdf@redhat.com>
Subject: Re: [PATCH for-3.1 v10 05/31] block: Respect backing bs in
 bdrv_refresh_filename
References: <20180809213528.14738-1-mreitz@redhat.com>
 <20180809213528.14738-6-mreitz@redhat.com>
 <w517ekasjtr.fsf@maestria.local.igalia.com>
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On 2018-08-28 15:26, Alberto Garcia wrote:
> On Thu 09 Aug 2018 11:35:02 PM CEST, Max Reitz wrote:
>> +    backing_overridden =3D bdrv_backing_overridden(bs);
>> +
>> +    if (bs->open_flags & BDRV_O_NO_IO) {
>> +        /* Without I/O, the backing file does not change anything.
>> +         * Therefore, in such a case (primarily qemu-img), we can
>> +         * pretend the backing file has not been overridden even if
>> +         * it technically has been. */
>> +        backing_overridden =3D false;
>> +    }
>=20
> What is this for?

qemu-img info opens files with BDRV_O_NO_BACKING.  Without this check,
you'd get a json:{} filename with "backing": null.

Max


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