Re: [PATCH 06/22] block: Add bdrv_recurse_can_replace()

From: Max Reitz <mreitz@redhat.com>
To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	"qemu-block@nongnu.org" <qemu-block@nongnu.org>
Cc: Kevin Wolf <kwolf@redhat.com>, Alberto Garcia <berto@igalia.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Subject: Re: [PATCH 06/22] block: Add bdrv_recurse_can_replace()
Date: Thu, 26 Sep 2019 13:03:55 +0200	[thread overview]
Message-ID: <3cdf9ba3-d79c-b62b-cccf-3fd444c0e252@redhat.com> (raw)
In-Reply-To: <37b5883c-b4e9-4b9d-fae4-8a1f0b04cd54@virtuozzo.com>

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On 25.09.19 14:39, Vladimir Sementsov-Ogievskiy wrote:
> 20.09.2019 18:27, Max Reitz wrote:
>> After a couple of follow-up patches, this function will replace
>> bdrv_recurse_is_first_non_filter() in check_to_replace_node().
>>
>> bdrv_recurse_is_first_non_filter() is both not sufficiently specific for
>> check_to_replace_node() (it allows cases that should not be allowed,
>> like replacing child nodes of quorum with dissenting data that have more
>> parents than just quorum), and it is too restrictive (it is perfectly
>> fine to replace filters).
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   include/block/block_int.h | 10 ++++++++++
>>   block.c                   | 38 ++++++++++++++++++++++++++++++++++++++
>>   2 files changed, 48 insertions(+)
>>
>> diff --git a/include/block/block_int.h b/include/block/block_int.h
>> index 5fd4f17d93..0be7d12f04 100644
>> --- a/include/block/block_int.h
>> +++ b/include/block/block_int.h
>> @@ -103,6 +103,13 @@ struct BlockDriver {
>>        */
>>       bool (*bdrv_recurse_is_first_non_filter)(BlockDriverState *bs,
>>                                                BlockDriverState *candidate);
>> +    /*
>> +     * Return true if @to_replace can be replaced by a BDS with the
>> +     * same data as @bs without it affecting @bs's behavior (that is,
>> +     * without it being visible to @bs's parents).
>> +     */
>> +    bool (*bdrv_recurse_can_replace)(BlockDriverState *bs,
>> +                                     BlockDriverState *to_replace);
>>   
>>       int (*bdrv_probe)(const uint8_t *buf, int buf_size, const char *filename);
>>       int (*bdrv_probe_device)(const char *filename);
>> @@ -1254,6 +1261,9 @@ void bdrv_format_default_perms(BlockDriverState *bs, BdrvChild *c,
>>                                  uint64_t perm, uint64_t shared,
>>                                  uint64_t *nperm, uint64_t *nshared);
>>   
>> +bool bdrv_recurse_can_replace(BlockDriverState *bs,
>> +                              BlockDriverState *to_replace);
>> +
>>   /*
>>    * Default implementation for drivers to pass bdrv_co_block_status() to
>>    * their file.
>> diff --git a/block.c b/block.c
>> index 7d99ca692c..a2deca4ac9 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -6206,6 +6206,44 @@ bool bdrv_recurse_is_first_non_filter(BlockDriverState *bs,
>>       return false;
>>   }
>>   
>> +/*
>> + * This function checks whether the given @to_replace is allowed to be
>> + * replaced by a node that always shows the same data as @bs.  This is
>> + * used for example to verify whether the mirror job can replace
>> + * @to_replace by the target mirrored from @bs.
>> + * To be replaceable, @bs and @to_replace may either be guaranteed to
>> + * always show the same data (because they are only connected through
>> + * filters), or some driver may allow replacing one of its children
>> + * because it can guarantee that this child's data is not visible at
>> + * all (for example, for dissenting quorum children that have no other
>> + * parents).
>> + */
>> +bool bdrv_recurse_can_replace(BlockDriverState *bs,
>> +                              BlockDriverState *to_replace)
>> +{
>> +    if (!bs || !bs->drv) {
>> +        return false;
>> +    }
>> +
>> +    if (bs == to_replace) {
>> +        return true;
>> +    }
>> +
>> +    /* For filters, we can recurse on our own */
>> +    if (bs->drv->is_filter) {
>> +        BdrvChild *child = bs->file ?: bs->backing;
> 
> then, maybe asset(!bs->drv->bdrv_recurse_can_replace)

It’s actually the other way around.  As you find yourself, blkverify is
a filter and has its own implementation.  That is entirely correct
because we cannot recurse to just bs->file in blkverify's case.  So we
should first invoke the driver-specific function, and then have the
generic filter code.

Max

>> +        return bdrv_recurse_can_replace(child->bs, to_replace);
>> +    }
> 
> or, this may be filter-skipping loop instead of recursion, like
> 
> while (bs && bs->drv && bs->drv->is_filter) {
>    bs = (bs->file ?: bs->backing)->bs;
> }
> 
> at function start.
> 
> either way:
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> 
>> +
>> +    /* See what the driver can do */
>> +    if (bs->drv->bdrv_recurse_can_replace) {
>> +        return bs->drv->bdrv_recurse_can_replace(bs, to_replace);
>> +    }
>> +
>> +    /* Safe default */
>> +    return false;
>> +}
>> +
>>   BlockDriverState *check_to_replace_node(BlockDriverState *parent_bs,
>>                                           const char *node_name, Error **errp)
>>   {
>>
> 
> 

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