24.04.2019 19:36, Max Reitz wrote: > On 19.04.19 12:23, Vladimir Sementsov-Ogievskiy wrote: >> 17.04.2019 19:22, Max Reitz wrote: >>> On 16.04.19 12:02, Vladimir Sementsov-Ogievskiy wrote: >>>> 10.04.2019 23:20, Max Reitz wrote: >>>>> What bs->file and bs->backing mean depends on the node. For filter >>>>> nodes, both signify a node that will eventually receive all R/W >>>>> accesses. For format nodes, bs->file contains metadata and data, and >>>>> bs->backing will not receive writes -- instead, writes are COWed to >>>>> bs->file. Usually. >>>>> >>>>> In any case, it is not trivial to guess what a child means exactly with >>>>> our currently limited form of expression. It is better to introduce >>>>> some functions that actually guarantee a meaning: >>>>> >>>>> - bdrv_filtered_cow_child() will return the child that receives requests >>>>> filtered through COW. That is, reads may or may not be forwarded >>>>> (depending on the overlay's allocation status), but writes never go >>>>> to >>>>> this child. >>>>> >>>>> - bdrv_filtered_rw_child() will return the child that receives requests >>>>> filtered through some very plain process. Reads and writes issued to >>>>> the parent will go to the child as well (although timing, etc. may be >>>>> modified). >>>>> >>>>> - All drivers but quorum (but quorum is pretty opaque to the general >>>>> block layer anyway) always only have one of these children: All read >>>>> requests must be served from the filtered_rw_child (if it exists), so >>>>> if there was a filtered_cow_child in addition, it would not receive >>>>> any requests at all. >>>>> (The closest here is mirror, where all requests are passed on to the >>>>> source, but with write-blocking, write requests are "COWed" to the >>>>> target. But that just means that the target is a special child that >>>>> cannot be introspected by the generic block layer functions, and that >>>>> source is a filtered_rw_child.) >>>>> Therefore, we can also add bdrv_filtered_child() which returns that >>>>> one child (or NULL, if there is no filtered child). >>>>> >>>>> Also, many places in the current block layer should be skipping filters >>>>> (all filters or just the ones added implicitly, it depends) when going >>>>> through a block node chain. They do not do that currently, but this >>>>> patch makes them. >>>>> >>>>> One example for this is qemu-img map, which should skip filters and only >>>>> look at the COW elements in the graph. The change to iotest 204's >>>>> reference output shows how using blkdebug on top of a COW node used to >>>>> make qemu-img map disregard the rest of the backing chain, but with this >>>>> patch, the allocation in the base image is reported correctly. >>>>> >>>>> Furthermore, a note should be made that sometimes we do want to access >>>>> bs->backing directly. This is whenever the operation in question is not >>>>> about accessing the COW child, but the "backing" child, be it COW or >>>>> not. This is the case in functions such as bdrv_open_backing_file() or >>>>> whenever we have to deal with the special behavior of @backing as a >>>>> blockdev option, which is that it does not default to null like all >>>>> other child references do. >>>>> >>>>> Finally, the query functions (query-block and query-named-block-nodes) >>>>> are modified to return any filtered child under "backing", not just >>>>> bs->backing or COW children. This is so that filters do not interrupt >>>>> the reported backing chain. This changes the output of iotest 184, as >>>>> the throttled node now appears as a backing child. >>>>> >>>>> Signed-off-by: Max Reitz <mre...@redhat.com> >>>>> --- >>>>> qapi/block-core.json | 4 + >>>>> include/block/block.h | 1 + >>>>> include/block/block_int.h | 40 +++++-- >>>>> block.c | 210 +++++++++++++++++++++++++++------ >>>>> block/backup.c | 8 +- >>>>> block/block-backend.c | 16 ++- >>>>> block/commit.c | 33 +++--- >>>>> block/io.c | 45 ++++--- >>>>> block/mirror.c | 21 ++-- >>>>> block/qapi.c | 30 +++-- >>>>> block/stream.c | 13 +- >>>>> blockdev.c | 88 +++++++++++--- >>>>> migration/block-dirty-bitmap.c | 4 +- >>>>> nbd/server.c | 6 +- >>>>> qemu-img.c | 29 ++--- >>>>> tests/qemu-iotests/184.out | 7 +- >>>>> tests/qemu-iotests/204.out | 1 + >>>>> 17 files changed, 411 insertions(+), 145 deletions(-) >>>> >>>> really huge... didn't you consider conversion file-by-file? >>> >>> Frankly, no, I just didn’t consider it. >>> >>> Hm. I don’t know, 30-patch series always look so frightening. >>> >>>>> diff --git a/block.c b/block.c >>>>> index 16615bc876..e8f6febda0 100644 >>>>> --- a/block.c >>>>> +++ b/block.c >>>> >>>> [..] >>>> >>>>> >>>>> @@ -3467,14 +3469,17 @@ static int >>>>> bdrv_reopen_parse_backing(BDRVReopenState *reopen_state, >>>>> /* >>>>> * Find the "actual" backing file by skipping all links that point >>>>> * to an implicit node, if any (e.g. a commit filter node). >>>>> + * We cannot use any of the bdrv_skip_*() functions here because >>>>> + * those return the first explicit node, while we are looking for >>>>> + * its overlay here. >>>>> */ >>>>> overlay_bs = bs; >>>>> - while (backing_bs(overlay_bs) && backing_bs(overlay_bs)->implicit) { >>>>> - overlay_bs = backing_bs(overlay_bs); >>>>> + while (overlay_bs->backing && >>>>> bdrv_filtered_bs(overlay_bs)->implicit) { >>>> >>>> So, you don't want to skip implicit filters with 'file' child? Then, why >>>> not to use >>>> child_bs(overlay_bs->backing), like in following if condition? >>> >>> I think it was an artifact of writing the patch. I started with >>> bdrv_filtered_bs() and then realized this depends on ->backing, >>> actually. There was no functional difference so I left it as it was. >>> >>> But you’re right, it is more clear to use child_bs(overlay_bs->backing) >>> isntead. >>> >>>> Could we instead make backing-based filters equal to file-based, to make >>>> it possible >>>> to use file-based filters in backing-chain related scenarios (like >>>> upcoming copy-on-read >>>> filter for stream)? So, to expand backing-chain concept to include filters >>>> with file child? >>> >>> If I understand you correctly, that’s basically the purpose of this >>> series and especially this patch here. As far as it is possible and >>> reasonable, I want filters that use bs->backing and bs->file behave the >>> same. >>> >>> However, there are cases where this is not possible and >>> bdrv_reopen_parse_backing() is one such case. bs->backing and bs->file >>> correspond to QAPI names, namely 'backing' and 'file'. If that >>> distinction was already visible to the user, we cannot change it now. >>> >>> We definitely cannot make file-based filters use bs->backing now because >>> you can create them over QAPI and they use 'file' as their child name. >>> Can we make backing-based filters use bs->file? Seems more likely, >>> because all of them are implicit nodes, so the user usually doesn’t see >>> them. But usually isn’t always; they do become user-visible once the >>> user specifies a node-name for mirror or commit. >>> >>> I found it more reasonable to introduce new functions that explicitly >>> express what kind of child they expect and then apply them everywhere as >>> I saw fit, instead of making the mirror/commit filter drivers use >>> bs->file and hope it works; not least because I’d still have to go >>> through the whole block layer and check every instance of bs->backing to >>> see whether it really needs bs->backing or whether it should use either >>> of bs->backing or bs->file. >>> >>>>> + overlay_bs = bdrv_filtered_bs(overlay_bs); >>>>> } >>>>> >>>>> /* If we want to replace the backing file we need some extra >>>>> checks */ >>>>> - if (new_backing_bs != backing_bs(overlay_bs)) { >>>>> + if (new_backing_bs != child_bs(overlay_bs->backing)) { > >>>>> /* Check for implicit nodes between bs and its backing file */ >>>>> if (bs != overlay_bs) { >>>>> error_setg(errp, "Cannot change backing link if '%s' has " >>>> >>>> [..] >>>> >>>>> @@ -4203,8 +4208,8 @@ int bdrv_change_backing_file(BlockDriverState *bs, >>>>> BlockDriverState *bdrv_find_overlay(BlockDriverState *active, >>>>> BlockDriverState *bs) >>>>> { >>>>> - while (active && bs != backing_bs(active)) { >>>>> - active = backing_bs(active); >>>>> + while (active && bs != bdrv_filtered_bs(active)) { >>>> >>>> hmm and here you actually support backing-chain with file-child-based >>>> filters in it.. >>> >>> Yes, because this is not about the QAPI 'backing' link. This function >>> should continue to work even if there are filters in the backing chain. >> >> this is a generic function to find overlay in backing chain and it may be >> used from different places, >> for example it is used in Andrey's series about filter for block-stream. > > Well, all places that use it accept backing chains with filters inside > of them. > >> It is used from qmp_block_commit, isn't it about QAPI? > > By "QAPI 'backing' link" I mean the user-visible block graph. Hm. I > wrote in my other mail that you could use query-named-block-nodes to see > that graph; apparently you can’t. So besides x-debug-query-block-graph, > we still don’t have any facility to query the block graph? I don’t know > what to say. > > Anyway, you can still construct the graph with blockdev-add, so it is > user-visible. And in that block graph, there is a 'backing' link, and > there is a 'file' link -- this is what I mean with "QAPI link". > > We have commands that are abstract and don’t work on specific graph > links. For instance, block-commit commits across a backing chain, so it > doesn’t matter whether the graph link is called 'backing' or whatever, > what is important is that it’s a COW link. But we should also ignore > filters on the way, so this patch makes block-commit and others use > those more abstract child access functions. > > But whenever it is about exactly the "file" or the "backing" link, we > have to use bs->file and bs->backing, respectively. That's just how it > currently is. > >>>>> + active = bdrv_filtered_bs(active); >>>>> } >>>>> >>>>> return active; >>>>> @@ -4226,11 +4231,11 @@ bool >>>>> bdrv_is_backing_chain_frozen(BlockDriverState *bs, BlockDriverState *base, >>>>> { >>>>> BlockDriverState *i; >>>>> >>>>> - for (i = bs; i != base; i = backing_bs(i)) { >>>>> + for (i = bs; i != base; i = child_bs(i->backing)) { >>>> >>>> and here don't.. >>> >>> Yes, because this function is about the QAPI 'backing' link. >> >> And this again a generic thing, that may be used in same places as >> bdrv_find_overlay, > > But it isn’t. > >> and it is used in series about block-stream filter too. So, for further >> developments >> we'll have to keep in mind all these differences between generic block layer >> functions, >> which supports .file children inside backing chain and which are not... > > I was wrong about bdrv_is_backing_chain_frozen(), if that helps (as I > wrote in my other (previous) mail). > > But for example bdrv_set_backing_hd() always has to use bs->backing, > because that’s what it’s about (and I do change its descriptive comment > to reflect that, so you don’t need to keep it in mind). Same for > bdrv_open_backing_file(). > > Hm, what other cases are there... > > bdrv_reopen_parse_backing(): Fundamentally, this too is about the > user-visible "backing" link (as specified through x-blockdev-reopen). > But the loop it contains is more difficult to translate than I had > thought. At some point, there needs to be a bs->backing link, because > that is what this function is about, but it should also skip all > implicit filters in the way, I think. So e.g. this should be recognized: > > bs ---backing--> COR ---file--> base > > @overlay_bs should be COR, I think...? I mean, as long as COR is an > implicit node. So the loop really should use bdrv_filtered_bs() > everywhere, and then the same afterwards. I think that we should also > ensure that @bs can support a ->backing child, but how would I check > that? Maybe it’s safe to just omit such a check... > > But then another issue comes in: The link to replace (in the above case > from "COR" to "base") is no longer necessarily a backing link. So > bdrv_reopen_commit() has to be capable of replacing both bs->backing and > bs->file. > > Actually, how does bdrv_reopen_commit() handle implicit nodes at all? > bdrv_reopen_parse_backing() just sets reopen_state->replace_backing_bs > and ->new_backing_bs. It doesn’t communicate anything about overlay_bs. > bdrv_reopen_commit() then asserts that !bs->backing->bs->implicit and > replaces bs->backing. So it seems to just fail on the implicit nodes > that bdrv_reopen_parse_backing() took care to skip... > > > OK, what else... bdrv_reopen_prepare() checks > reopen_state->bs->backing, which I claim is correct because while there > may be implicit filters in the chain, the first link has to be a > ->backing link.
[sorry for a long delay] Are you working on next version or waiting for more reviews? Why first link should be backing? We want to skip all implicit filters, including file-child-based in following call to bdrv_reopen_parse_backing(). So, don't we want something like bdrv_backing_chain_next() here? But then a question, could reopen_state->bs be filter itself... > > bdrv_backing_overridden() has to query bs->backing because this function > is used when it is about a specific characteristic of the backing link: > There is a non-null default (given by the image header), so if the > current bs->backing matches this default, you do not have to specify the > backing filename in either blockdev-add or a filename. Same in > bdrv_refresh_filename(). > > > I hope that was all...? > > Max > -- Best regards, Vladimir