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Re: [PATCH v3 09/33] block: Add generic bdrv_inherited_options()
From: |
Max Reitz |
Subject: |
Re: [PATCH v3 09/33] block: Add generic bdrv_inherited_options() |
Date: |
Thu, 7 May 2020 10:49:17 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 |
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 <address@hidden>
>> Reviewed-by: Eric Blake <address@hidden>
>> ---
>> 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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