Am 24.08.2020 um 16:41 hat Max Reitz geschrieben: > On 24.08.20 16:07, Kevin Wolf wrote: > > Am 24.08.2020 um 15:18 hat Max Reitz geschrieben: > >> On 21.08.20 17:50, Kevin Wolf wrote: > >>> Am 25.06.2020 um 17:22 hat Max Reitz geschrieben: > >>>> We have to perform an active commit whenever the top node has a parent > >>>> that has taken the WRITE permission on it. > >>>> > >>>> Signed-off-by: Max Reitz <[email protected]> > >>>> Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]> > >>>> --- > >>>> blockdev.c | 24 +++++++++++++++++++++--- > >>>> 1 file changed, 21 insertions(+), 3 deletions(-) > >>>> > >>>> diff --git a/blockdev.c b/blockdev.c > >>>> index 402f1d1df1..237fffbe53 100644 > >>>> --- a/blockdev.c > >>>> +++ b/blockdev.c > >>>> @@ -2589,6 +2589,7 @@ void qmp_block_commit(bool has_job_id, const char > >>>> *job_id, const char *device, > >>>> AioContext *aio_context; > >>>> Error *local_err = NULL; > >>>> int job_flags = JOB_DEFAULT; > >>>> + uint64_t top_perm, top_shared; > >>>> > >>>> if (!has_speed) { > >>>> speed = 0; > >>>> @@ -2704,14 +2705,31 @@ void qmp_block_commit(bool has_job_id, const > >>>> char *job_id, const char *device, > >>>> goto out; > >>>> } > >>>> > >>>> - if (top_bs == bs) { > >>>> + /* > >>>> + * Active commit is required if and only if someone has taken a > >>>> + * WRITE permission on the top node. > >>> > >>> ...or if someone wants to take a WRITE permission while the job is > >>> running. > >>> > >>> Future intentions of the user is something that we can't know, so maybe > >>> this should become an option in the future (not in this series, of > >>> course). > >>> > >>>> Historically, we have always > >>>> + * used active commit for top nodes, so continue that practice. > >>>> + * (Active commit is never really wrong.) > >>>> + */ > >>> > >>> Changing the practice would break compatibility with clients that start > >>> an active commit job and then attach it to a read-write device, so we > >>> must continue the practice. I think the comment should be clearer about > >>> this, it sounds more like "no reason, but why not". > >> > >> I think that’s what I meant by “historically”. Is “legacily” a word? > >> > >> But sure, I can make it more explicit. > >> > >>> This is even more problematic because the commit job doesn't unshare > >>> BLK_PERM_WRITE yet, so it would lead to silent corruption rather than an > >>> error. > >>> > >>>> + bdrv_get_cumulative_perm(top_bs, &top_perm, &top_shared); > >>>> + if (top_perm & BLK_PERM_WRITE || > >>>> + bdrv_skip_filters(top_bs) == bdrv_skip_filters(bs)) > >>>> + { > >>>> if (has_backing_file) { > >>>> error_setg(errp, "'backing-file' specified," > >>>> " but 'top' is the active layer"); > >>> > >>> Hm, this error message isn't accurate any more. > >>> > >>> In fact, the implementation isn't consistent with the QAPI documentation > >>> any more, because backing-file is only an error for the top level. > >> > >> Hm. I wanted to agree, and then I wanted to come up with a QAPI > >> documentation that fits the new behavior (because I think it makes more > >> sense to change the QAPI documentation along with the behavior change, > >> rather than to force us to allow backing-file for anything that isn’t on > >> the top layer). > >> > >> But in the process of coming up with a better description, I noticed > >> that this doesn’t say “is a root node”, it says “is the active layer”. > >> I would say a node in the active layer is a node that has some parent > >> that has taken a WRITE permission on it. So actually I think that the > >> documentation is right, and this code only now fits. > > > > Then you may have not only "the" active layer, but multiple active > > layers. I find this a bit counterintuitive. > > Depends on what you count as a layer. I don’t think that’s a clearly > defined term, is it? I only know of “active layer”, “format layer”, > “protocol layer”, and you can at least have multiple format layers above > each other. So I don’t find it counterintuitive. > > But perhaps it’d be best to just get away from the term “active layer”, > as you propose below.
Hm, if I needed to describe what a layer is for me intuitively, I guess it would be something like each non-filter node on a node chain with all of the filters directly on top of it? Depending on which link you follow, you get different sets of layers: For bs->file, you get the format/protocol layer distinction. For bs->backing, you get essentially what bdrv_backing_chain_next() iterates. In this context (which is talking about COW overlays), I expected the bs->backing link to apply. The active layer is then the COW layer that is directly referenced by a guest device, block job or block export. > > There is a simple reason why backing-file is an error for a root node: > > It doesn't have overlays, so a value to write to the header of overlay > > images just doesn't make sense. > > Ah, yeah... > > > The same reasoning doesn't apply for writable images that do have > > overlays. Forbidding backing-file is a more arbitrary restriction there. > > I'm not saying that we can't make arbitrary restrictions where allowing > > an option is not worth the effort, but I feel they should be spelt out > > more explicitly instead of twisting words like "active layer" until they > > fit the code. > > I’m all for spelling it out more explicitly. I just noticed that I > couldn’t clearly distinguish “active layer” from “other” cases of nodes > with writers on them, which is why I noted that “active” to me means the > post-patch behavior already. > > You’re right that there is no semantic reason for making it an error. > So I just want it to be an error to be lazy. I hope you let me do that. > (I don’t think there’s much of a problem with it, considering that > commits on nodes that have the WRITE permission taken are basically just > completely broken right now.) That I'm happy to allow you to be lazy in this case is what I wanted to express with "I'm not saying that we can't make arbitrary restrictions". :-) > >> Though I do think this wants for some clarification. Perhaps “If 'top' > >> is the active layer (i.e., is a node that may be written to), specifying > >> a backing [...]”? > > > > "If 'top' doesn't have an overlay image or is in use by a writer..."? > > I.e., avoiding the term “active layer” altogether? Sounds good. Only, > I don’t know about “writer”... But it’s already used in > BlockdevOptionsFile.dynamic-auto-read-only’s description, so I suppose > we can use it here, too. (I just don’t know if as a > non-block-layer-developer I’d know what it means.) I was thinking of something like "is used read-write" at first, but then realised that write-only is possible, too, so it wouldn't be entirely accurate... > (Also, yes, you’re right, the current behavior of giving all root nodes > an active commit of course remains, even when there are no writers.) > > >> There’s more wrong with the specification, namely the whole part under > >> @backing-file past the “(Since 2.1)”, starting with “If top == base”. I > >> think all of that should go to the top level. (And “If top == active” > >> should be changed to “If top is active (i.e., may be written to)”.) > > > > At least the latter only becomes wrong with this patch, so I think it > > needs to be changed by this patch. > > Sure. So I understand you agree with moving the whole chunk, right? I don't mind either way. Kevin
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