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

[Stefano Garzarella]

On Fri, Jul 26, 2019 at 12:58:58PM +0200, Max Reitz wrote:
> 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 <address@hidden>
> >> ---
> >>  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().

Ah right! And eventually a bug in .bdrv_has_zero_init() would be masked.

So,
Reviewed-by: Stefano Garzarella <address@hidden>

Thanks,
Stefano