Re: [PATCH v3 09/33] block: Add generic bdrv_inherited_options()

[Max Reitz <mreitz@redhat.com>]

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On 06.05.20 12:37, Kevin Wolf wrote:
> Am 18.02.2020 um 13:42 hat Max Reitz geschrieben:
>> After the series this patch belongs to, we want to have a common
>> BdrvChildClass that encompasses all of child_file, child_format, and
>> child_backing.  Such a single class needs a single .inherit_options()
>> implementation, and this patch introduces it.
>>
>> The next patch will show how the existing implementations can fall back
>> to it just by passing appropriate BdrvChildRole and parent_is_format
>> values.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> Reviewed-by: Eric Blake <eblake@redhat.com>
>> ---
>>  block.c | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 84 insertions(+)
>>
>> diff --git a/block.c b/block.c
>> index c33f0e9b42..9179b9b604 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -998,6 +998,90 @@ static void bdrv_temp_snapshot_options(int *child_flags, QDict *child_options,
>>      *child_flags &= ~BDRV_O_NATIVE_AIO;
>>  }
>>  
>> +/*
>> + * Returns the options and flags that a generic child of a BDS should
>> + * get, based on the given options and flags for the parent BDS.
>> + */
>> +static void __attribute__((unused))
>> +    bdrv_inherited_options(BdrvChildRole role, bool parent_is_format,
>> +                           int *child_flags, QDict *child_options,
>> +                           int parent_flags, QDict *parent_options)
>> +{
>> +    int flags = parent_flags;
>> +
>> +    /*
>> +     * First, decide whether to set, clear, or leave BDRV_O_PROTOCOL.
>> +     * Generally, the question to answer is: Should this child be
>> +     * format-probed by default?
>> +     */
>> +
>> +    /*
>> +     * Pure and non-filtered data children of non-format nodes should
>> +     * be probed by default (even when the node itself has BDRV_O_PROTOCOL
>> +     * set).  This only affects a very limited set of drivers (namely
>> +     * quorum and blkverify when this comment was written).
>> +     * Force-clear BDRV_O_PROTOCOL then.
>> +     */
>> +    if (!parent_is_format &&
>> +        (role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA |
>> +                 BDRV_CHILD_FILTERED)) ==
>> +            BDRV_CHILD_DATA)
> 
> You could avoid the odd indentation (I can't decide whether or not it
> should be one space more to align correctly) and probably also make the
> expression more readable if you split it into:
> 
>     (role & BDRV_CHILD_DATA) &&
>     !(role & (BDRV_CHILD_METADATA | BDRV_CHILD_FILTERED))

Yes, looks good.

>> +    {
>> +        flags &= ~BDRV_O_PROTOCOL;
>> +    }
>> +
>> +    /*
>> +     * All children of format nodes (except for COW children) and all
>> +     * metadata children in general should never be format-probed.
>> +     * Force-set BDRV_O_PROTOCOL then.
>> +     */
>> +    if ((parent_is_format && !(role & BDRV_CHILD_COW)) ||
>> +        (role & BDRV_CHILD_METADATA))
>> +    {
>> +        flags |= BDRV_O_PROTOCOL;
>> +    }
>> +
>> +    /*
>> +     * If the cache mode isn't explicitly set, inherit direct and no-flush from
>> +     * the parent.
>> +     */
>> +    qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_DIRECT);
>> +    qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_NO_FLUSH);
>> +    qdict_copy_default(child_options, parent_options, BDRV_OPT_FORCE_SHARE);
>> +
>> +    if (role & BDRV_CHILD_COW) {
>> +        /* backing files are always opened read-only */
> 
> Not "always", just by default.

OK.  I just copied the comment from bdrv_backing_options().

>> +        qdict_set_default_str(child_options, BDRV_OPT_READ_ONLY, "on");
>> +        qdict_set_default_str(child_options, BDRV_OPT_AUTO_READ_ONLY, "off");
>> +    } else {
>> +        /* Inherit the read-only option from the parent if it's not set */
>> +        qdict_copy_default(child_options, parent_options, BDRV_OPT_READ_ONLY);
>> +        qdict_copy_default(child_options, parent_options,
>> +                           BDRV_OPT_AUTO_READ_ONLY);
>> +    }
>> +
>> +    if (parent_is_format && !(role & BDRV_CHILD_COW)) {
>> +        /*
>> +         * Our format drivers take care to send flushes and respect
>> +         * unmap policy, so we can default to enable both on lower
>> +         * layers regardless of the corresponding parent options.
>> +         */
>> +        qdict_set_default_str(child_options, BDRV_OPT_DISCARD, "unmap");
>> +    }
> 
> Why the restriction to format here? Don't we break "unmap" propagation
> through filters with this?

Right now (before this series), the behavior seems ambiguous, in that
for filters that use bs->file, it is set, but for those that use
bs->backing, it isn’t.

But I suspect the main reason for what I did is the way I interpreted
the comment (which before this series only mentions block drivers in
general, not specifically format drivers): It sounded to me as if the
block driver needed to respect the unmap policy, and I didn’t think
filters did that.  So it was my understanding that filter drivers would
just propagate discards and thus we couldn’t default-enable unmap on
their children.

But I was wrong, the block driver doesn’t need to respect anything,
because bdrv_co_pdiscard() already does.

So I suppose it should indeed be enabled for all children, with the
comment changed to express that it isn’t any block driver that respects
unmap policy, but bdrv_co_pdiscard(), e.g.:

bdrv_co_pdiscard() respects unmap policy for the parent, so we can
default to enable it on lower layers regardless of the parent option.

> It would probably also be a good question why we don't propagate it to
> the backing file, but this is preexisting.

I suppose we should, although it’s irrelevant, so.  I suppose I’ll just
drop the parent_is_format, adjust the comment and that should be fine
for this series.

Max


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