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 <mre...@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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