Am 28.04.2020 um 13:08 hat Max Reitz geschrieben: > On 28.04.20 10:55, Vladimir Sementsov-Ogievskiy wrote: > > Hi! > > > > I wanted to resend my "[PATCH 0/4] fix & merge block_status_above and > > is_allocated_above", and returned to all the inconsistencies about > > block-status. I keep in mind Max's series about child-access functions, > > and Andrey's work about using COR filter in block-stream, which depends > > on Max's series (because, without them COR fitler with file child breaks > > backing chains).. And, it seems that it's better to discuss some > > questions before resending. > > > > First, problems about block-status: > > > > 1. We consider ALLOCATED = ZERO | DATA, and documented as follows: > > > > * BDRV_BLOCK_DATA: allocation for data at offset is tied to this layer > > * BDRV_BLOCK_ZERO: offset reads as zero > > * BDRV_BLOCK_OFFSET_VALID: an associated offset exists for accessing > > raw data > > * BDRV_BLOCK_ALLOCATED: the content of the block is determined by this > > * layer rather than any backing, set by block > > layer > > > > This actually means, that we should always have BDRV_BLOCK_ALLOCATED for > > formats which doesn't support backing. So, all such format drivers must > > return ZERO or DATA (or both?), yes?. Seems file-posix does so, but, for > > example, iscsi - doesn't. > > Hm. I could imagine that there are formats that have non-zero holes > (e.g. 0xff or just garbage). It would be a bit wrong for them to return > ZERO or DATA then. > > But OTOH we don’t care about such cases, do we? We need to know whether > ranges are zero, data, or unallocated. If they aren’t zero, we only > care about whether reading from it will return data from this layer or not. > > So I suppose that anything that doesn’t support backing files (or > filtered children) should always return ZERO and/or DATA.
I'm not sure I agree with the notion that everything should be BDRV_BLOCK_ALLOCATED at the lowest layer. It's not what it means today at least. If we want to change this, we will have to check all callers of bdrv_is_allocated() and friends who might use this to find holes in the file. Basically, the way bdrv_is_allocated() works today is that we assume an implicit zeroed backing layer even for block drivers that don't support backing files. Kevin
signature.asc
Description: PGP signature