Re: [Qemu-devel] [PATCH v2 03/11] block: Add bdrv_has_zero_init_truncate()

[Max Reitz <mreitz@redhat.com>]

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On 26.07.19 11:04, Stefano Garzarella wrote:
> On Wed, Jul 24, 2019 at 07:12:31PM +0200, Max Reitz wrote:
>> No .bdrv_has_zero_init() implementation returns 1 if growing the file
>> would add non-zero areas (at least with PREALLOC_MODE_OFF), so using it
>> in lieu of this new function was always safe.
>>
>> But on the other hand, it is possible that growing an image that is not
>> zero-initialized would still add a zero-initialized area, like when
>> using nonpreallocating truncation on a preallocated image.  For callers
>> that care only about truncation, not about creation with potential
>> preallocation, this new function is useful.
>>
>> Alternatively, we could have added a PreallocMode parameter to
>> bdrv_has_zero_init().  But the only user would have been qemu-img
>> convert, which does not have a plain PreallocMode value right now -- it
>> would have to parse the creation option to obtain it.  Therefore, the
>> simpler solution is to let bdrv_has_zero_init() inquire the
>> preallocation status and add the new bdrv_has_zero_init_truncate() that
>> presupposes PREALLOC_MODE_OFF.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>  include/block/block.h     |  1 +
>>  include/block/block_int.h |  7 +++++++
>>  block.c                   | 21 +++++++++++++++++++++
>>  3 files changed, 29 insertions(+)
>>
>> diff --git a/include/block/block.h b/include/block/block.h
>> index 50a07c1c33..5321d8afdf 100644
>> --- a/include/block/block.h
>> +++ b/include/block/block.h
>> @@ -438,6 +438,7 @@ int bdrv_pdiscard(BdrvChild *child, int64_t offset, int64_t bytes);
>>  int bdrv_co_pdiscard(BdrvChild *child, int64_t offset, int64_t bytes);
>>  int bdrv_has_zero_init_1(BlockDriverState *bs);
>>  int bdrv_has_zero_init(BlockDriverState *bs);
>> +int bdrv_has_zero_init_truncate(BlockDriverState *bs);
>>  bool bdrv_unallocated_blocks_are_zero(BlockDriverState *bs);
>>  bool bdrv_can_write_zeroes_with_unmap(BlockDriverState *bs);
>>  int bdrv_block_status(BlockDriverState *bs, int64_t offset,
>> diff --git a/include/block/block_int.h b/include/block/block_int.h
>> index 6a0b1b5008..d7fc6b296b 100644
>> --- a/include/block/block_int.h
>> +++ b/include/block/block_int.h
>> @@ -420,9 +420,16 @@ struct BlockDriver {
>>      /*
>>       * Returns 1 if newly created images are guaranteed to contain only
>>       * zeros, 0 otherwise.
>> +     * Must return 0 if .bdrv_has_zero_init_truncate() returns 0.
>>       */
> 
> Does it make sense to make sure of that in the bdrv_has_zero_init()?
> 
> I mean something like this:
> 
> int bdrv_has_zero_init(BlockDriverState *bs)
> {
>     ...
>     if (bs->drv->bdrv_has_zero_init && bs->drv->bdrv_has_zero_init_truncate) {
>         return bs->drv->bdrv_has_zero_init_truncate(bs) &&
>                bs->drv->bdrv_has_zero_init(bs);
>     } else if (bs->drv->bdrv_has_zero_init)
>         return bs->drv->bdrv_has_zero_init(bs);
>     }
>     ...
> }

I thought about it, but I didn’t like it because that would mean that
bdrv_has_zero_init() kind of differs from .bdrv_has_zero_init().

Max

> Thanks,
> Stefano
> 
>>      int (*bdrv_has_zero_init)(BlockDriverState *bs);
>>  
>> +    /*
>> +     * Returns 1 if new areas added by growing the image with
>> +     * PREALLOC_MODE_OFF contain only zeros, 0 otherwise.
>> +     */
>> +    int (*bdrv_has_zero_init_truncate)(BlockDriverState *bs);
>> +
>>      /* Remove fd handlers, timers, and other event loop callbacks so the event
>>       * loop is no longer in use.  Called with no in-flight requests and in
>>       * depth-first traversal order with parents before child nodes.
>> diff --git a/block.c b/block.c
>> index cbd8da5f3b..81ae44dcf3 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -5066,6 +5066,27 @@ int bdrv_has_zero_init(BlockDriverState *bs)
>>      return 0;
>>  }
>>  
>> +int bdrv_has_zero_init_truncate(BlockDriverState *bs)
>> +{
>> +    if (!bs->drv) {
>> +        return 0;
>> +    }
>> +
>> +    if (bs->backing) {
>> +        /* Depends on the backing image length, but better safe than sorry */
>> +        return 0;
>> +    }
>> +    if (bs->drv->bdrv_has_zero_init_truncate) {
>> +        return bs->drv->bdrv_has_zero_init_truncate(bs);
>> +    }
>> +    if (bs->file && bs->drv->is_filter) {
>> +        return bdrv_has_zero_init_truncate(bs->file->bs);
>> +    }
>> +
>> +    /* safe default */
>> +    return 0;
>> +}
>> +
>>  bool bdrv_unallocated_blocks_are_zero(BlockDriverState *bs)
>>  {
>>      BlockDriverInfo bdi;
>> -- 
>> 2.21.0
>>
> 



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