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block: Add bdrv_has_zero_init_truncate()

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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>
Message-id: 20190724171239.8764-4-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Max Reitz 2019-07-24 19:12:31 +02:00
parent cdf3bc934a
commit ceaca56fee
3 changed files with 29 additions and 0 deletions
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21
block.c
View File

@ -5078,6 +5078,27 @@ 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;

1
include/block/block.h
View File

@ -443,6 +443,7 @@ 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,

7
include/block/block_int.h
View File

@ -419,9 +419,16 @@ 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.
*/
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.