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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <344eec5c-8908-7b32-5d5f-61911253a621@virtuozzo.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="eMorwZOVWopjgZeMHJ77chDJmeBPGtSji" X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Tue, 07 May 2019 15:13:24 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 X-Content-Filtered-By: Mailman/MimeDel 2.1.21 Subject: Re: [Qemu-devel] [PATCH v4 02/11] block: Filtered children access functions X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , "qemu-devel@nongnu.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --eMorwZOVWopjgZeMHJ77chDJmeBPGtSji From: Max Reitz To: Vladimir Sementsov-Ogievskiy , "qemu-block@nongnu.org" Cc: Kevin Wolf , "qemu-devel@nongnu.org" Message-ID: <5794529f-9451-4dd9-c509-07df5cefdead@redhat.com> Subject: Re: [Qemu-devel] [PATCH v4 02/11] block: Filtered children access functions References: <20190410202033.28617-1-mreitz@redhat.com> <20190410202033.28617-3-mreitz@redhat.com> <344eec5c-8908-7b32-5d5f-61911253a621@virtuozzo.com> In-Reply-To: <344eec5c-8908-7b32-5d5f-61911253a621@virtuozzo.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On 07.05.19 15:30, 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 wit= h >> 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 reques= ts >> 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 request= s >> 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 rea= d >> 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 th= e >> source, but with write-blocking, write requests are "COWed" to the >> target. But that just means that the target is a special child tha= t >> cannot be introspected by the generic block layer functions, and th= at >> 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 filter= s >> (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 on= ly >> 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 th= is >> 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 n= ot >> 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() o= r >> 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 >> --- >> 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(-) >> >> diff --git a/qapi/block-core.json b/qapi/block-core.json >> index 7ccbfff9d0..dbd9286e4a 100644 >> --- a/qapi/block-core.json >> +++ b/qapi/block-core.json >> @@ -2502,6 +2502,10 @@ >> # On successful completion the image file is updated to drop the bac= king file >> # and the BLOCK_JOB_COMPLETED event is emitted. >> # >> +# In case @device is a filter node, block-stream modifies the first n= on-filter >> +# overlay node below it to point to base's backing node (or NULL if @= base was >> +# not specified) instead of modifying @device itself. >> +# >=20 > Is it necessary, why we can't keep it as is, modifying exactly device n= ode? May be, > user wants to use filter in stream process, throttling for example. That wouldn't make any sense. Say you have this configuration: throttle -> top -> base Now you stream from base to throttle. The data goes from base through throttle to top. You propose to then make throttle point to base: throttle -> base This will discard all the data in top. Filters don=E2=80=99t store any data. You need to keep the top data stor= ing image, i.e. the first non-filter overlay. >> # @job-id: identifier for the newly-created block job. If >> # omitted, the device name will be used. (Since 2.7) >> # [...] >> @@ -2345,7 +2347,7 @@ void bdrv_set_backing_hd(BlockDriverState *bs, B= lockDriverState *backing_hd, >> bool update_inherits_from =3D bdrv_chain_contains(bs, backing_hd= ) && >> bdrv_inherits_from_recursive(backing_hd, bs); >> =20 >> - if (bdrv_is_backing_chain_frozen(bs, backing_bs(bs), errp)) { >> + if (bdrv_is_backing_chain_frozen(bs, child_bs(bs->backing), errp)= ) { >=20 > If we support file-filters for frozen backing chain, could it go throug= h file child here? > Hmm, only in case when we are going to set backing hd for file-filter..= Hmm, could filter have > both file and backing children? No. A filter passes through data from its children, so it can only have a single child, or it is quorum. The file/backing combination is reserved for COW overlays. file is where the current layer=E2=80=99s data is, backing is the filtered child.= > Your new API don't restrict it, and choses backing as a default > in this case in bdrv_filtered_rw_child(), so, I assume you suppose poss= ibility of it. I can add an assertion against it if you=E2=80=99d like. > Here we don't want to check the chain, we exactly want to check backing= link, so it should be > something like >=20 > if (bs->backing && bs->backing->frozen) { > error_setg("backig exists and frozen!"); > return; > } >=20 >=20 > Hmm, on the other hand, if we have frozen backing chain, going through = file child, we must not add > backing child to the node with file child, as it will change backing ch= ain (which by default goes > through backing).. >=20 > Anyway, we don't need to check the whole backing chain, as we may find = other frozen backing subchain, > far away of bs.. So, we possibly want to check >=20 > if (bdrv_filtered_child(bs) && bdrv_filtered_child(bs)->frozed) { > ERROR > } >=20 >=20 > .... >=20 > also, we'll need to check for frozen file child, when we want to replac= e it. I don=E2=80=99t quite understand. It sounds to me like you=E2=80=99re sa= ying we don=E2=80=99t need to check the whole chain here but just the immediate child. But isn=E2=80=99t that true regardless of this series? >> return; >> } >> =20 [...] >> @@ -3634,7 +3639,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_= state, BlockReopenQueue *queue, >> * its metadata. Otherwise the 'backing' option can be omitted. >> */ >> if (drv->supports_backing && reopen_state->backing_missing && >> - (backing_bs(reopen_state->bs) || reopen_state->bs->backing_fi= le[0])) { >> + (reopen_state->bs->backing || reopen_state->bs->backing_file[= 0])) { >=20 > and if we skip implicit filters in bdrv_backing_chain_next(), shouldn't= we skip them > here too? This is a check whether it is mandatory for the user to specify the 'backing' key when reopening @bs. It is mandatory if it currently has a backing node, or if it should get a backing file by default because the image header says so. We don=E2=80=99t care about any node in particular. The question is just= =E2=80=9CIs there a backing node?=E2=80=9D. So I don=E2=80=99t see how skipping filt= ers would change anything. >> error_setg(errp, "backing is missing for '%s'", >> reopen_state->bs->node_name); >> ret =3D -EINVAL; >> @@ -3779,7 +3784,7 @@ void bdrv_reopen_commit(BDRVReopenState *reopen_= state) >> * from bdrv_set_backing_hd()) has the new values. >> */ >> if (reopen_state->replace_backing_bs) { >> - BlockDriverState *old_backing_bs =3D backing_bs(bs); >> + BlockDriverState *old_backing_bs =3D child_bs(bs->backing); >> assert(!old_backing_bs || !old_backing_bs->implicit); >> /* Abort the permission update on the backing bs we're detac= hing */ >> if (old_backing_bs) { >> @@ -4203,8 +4208,8 @@ int bdrv_change_backing_file(BlockDriverState *b= s, >> BlockDriverState *bdrv_find_overlay(BlockDriverState *active, >> BlockDriverState *bs) >> { >> - while (active && bs !=3D backing_bs(active)) { >> - active =3D backing_bs(active); >> + while (active && bs !=3D bdrv_filtered_bs(active)) { >=20 > need to adjust comment to the function then, as we may find file-based-= overlay, not backing. Yes, true. >> + active =3D bdrv_filtered_bs(active); >> } >> =20 >> return active; [...] >> @@ -4494,10 +4497,14 @@ bool bdrv_is_sg(BlockDriverState *bs) >> =20 >> bool bdrv_is_encrypted(BlockDriverState *bs) >> { >> - if (bs->backing && bs->backing->bs->encrypted) { >> + BlockDriverState *filtered =3D bdrv_filtered_bs(bs); >> + if (bs->encrypted) { >> + return true; >> + } >> + if (filtered && bdrv_is_encrypted(filtered)) { >> return true; >> } >> - return bs->encrypted; >> + return false; >> } >=20 > one backing child -> recursion through extended backing chain Yes, but isn=E2=80=99t that what we want? [...] >> @@ -4866,20 +4887,24 @@ int bdrv_has_zero_init_1(BlockDriverState *bs)= >> =20 >> int bdrv_has_zero_init(BlockDriverState *bs) >> { >> + BlockDriverState *filtered; >> + >> if (!bs->drv) { >> return 0; >> } >> =20 >> /* If BS is a copy on write image, it is initialized to >> the contents of the base image, which may not be zeroes. */ >> - if (bs->backing) { >> + if (bdrv_filtered_cow_child(bs)) { >> return 0; >> } >> if (bs->drv->bdrv_has_zero_init) { >> return bs->drv->bdrv_has_zero_init(bs); >> } >> - if (bs->file && bs->drv->is_filter) { >> - return bdrv_has_zero_init(bs->file->bs); >> + >> + filtered =3D bdrv_filtered_rw_bs(bs); >> + if (filtered) { >> + return bdrv_has_zero_init(filtered); >> } >=20 > add recursion for filters Not really, we had that before. >> =20 >> /* safe default */ [...] >> diff --git a/block/backup.c b/block/backup.c >> index 9988753249..9c08353b23 100644 >> --- a/block/backup.c >> +++ b/block/backup.c >> @@ -577,6 +577,7 @@ BlockJob *backup_job_create(const char *job_id, Bl= ockDriverState *bs, >> int64_t len; >> BlockDriverInfo bdi; >> BackupBlockJob *job =3D NULL; >> + bool target_does_cow; >> int ret; >> =20 >> assert(bs); >> @@ -671,8 +672,9 @@ BlockJob *backup_job_create(const char *job_id, Bl= ockDriverState *bs, >> /* If there is no backing file on the target, we cannot rely on = COW if our >> * backup cluster size is smaller than the target cluster size. = Even for >> * targets with a backing file, try to avoid COW if possible. */= >> + target_does_cow =3D bdrv_filtered_cow_child(target); >=20 > So, you excluded false-positive case when target is backing-filter. I t= hink, we'd better skip > filters here: >=20 > target_does_cow =3D bdrv_filtered_cow_child(bdrv_skip_rw_filters(target= )) Sounds correct, yes. I suppose we then need a fix for compression, too, though. Currently, the code checks whether the target driver offers =2Ebdrv_co_pwritev_compressed(). First, we should do the same there and skip filters. But second, in block/io.c, we then need to also skip filters for compressed writes (by issuing normal writes to them with the BDRV_REQ_WRITE_COMPRESSED flag set). >> ret =3D bdrv_get_info(target, &bdi); >> - if (ret =3D=3D -ENOTSUP && !target->backing) { >> + if (ret =3D=3D -ENOTSUP && !target_does_cow) { >> /* Cluster size is not defined */ >> warn_report("The target block device doesn't provide " >> "information about the block size and it doesn't= have a " [...] >> diff --git a/block/io.c b/block/io.c >> index dfc153b8d8..83c2b6b46a 100644 >> --- a/block/io.c >> +++ b/block/io.c [...] >> @@ -2208,7 +2218,7 @@ static int coroutine_fn bdrv_co_block_status_abo= ve(BlockDriverState *bs, >> bool first =3D true; >> =20 >> assert(bs !=3D base); >> - for (p =3D bs; p !=3D base; p =3D backing_bs(p)) { >> + for (p =3D bs; p !=3D base; p =3D bdrv_filtered_bs(p)) { >> ret =3D bdrv_co_block_status(p, want_zero, offset, bytes, pn= um, map, >> file); >=20 > Interesting that for filters who use bdrv_co_block_status_from_backing = and > bdrv_co_block_status_from_file we will finally call .bdrv_co_block_stat= us of > underalying real node two or more times.. It's not wrong but obviously = not optimal. Hm. If @p is a filter, we could skip straight to *file. Would that work= ? [...] >> diff --git a/block/mirror.c b/block/mirror.c >> index 8b2404051f..80cef587f0 100644 >> --- a/block/mirror.c >> +++ b/block/mirror.c >> @@ -660,8 +660,9 @@ static int mirror_exit_common(Job *job) >> &error_abort); >> if (!abort && s->backing_mode =3D=3D MIRROR_SOURCE_BACKING_CHAIN= ) { >> BlockDriverState *backing =3D s->is_none_mode ? src : s->bas= e; >> - if (backing_bs(target_bs) !=3D backing) { >> - bdrv_set_backing_hd(target_bs, backing, &local_err); >> + if (bdrv_backing_chain_next(target_bs) !=3D backing) { >> + bdrv_set_backing_hd(bdrv_skip_rw_filters(target_bs), back= ing, >=20 > hmm, here you support filters above target_bs ... >=20 >> + &local_err); >> if (local_err) { >> error_report_err(local_err); >> ret =3D -EPERM; >> @@ -711,7 +712,7 @@ static int mirror_exit_common(Job *job) >> block_job_remove_all_bdrv(bjob); >> bdrv_child_try_set_perm(mirror_top_bs->backing, 0, BLK_PERM_ALL,= >> &error_abort); >> - bdrv_replace_node(mirror_top_bs, backing_bs(mirror_top_bs), &erro= r_abort); >> + bdrv_replace_node(mirror_top_bs, mirror_top_bs->backing->bs, &err= or_abort); >> =20 >> /* We just changed the BDS the job BB refers to (with either or = both of the >> * bdrv_replace_node() calls), so switch the BB back so the clea= nup does >> @@ -903,7 +904,7 @@ static int coroutine_fn mirror_run(Job *job, Error= **errp) >> } else { >> s->target_cluster_size =3D BDRV_SECTOR_SIZE; >> } >> - if (backing_filename[0] && !target_bs->backing && >> + if (backing_filename[0] && !bdrv_filtered_cow_child(target_bs) &&= >=20 > ... and here - not Hm, yes, I think that is a mistake. I=E2=80=99ll try to fix it. > [stopped here for now] Thanks so far! :-) Max --eMorwZOVWopjgZeMHJ77chDJmeBPGtSji Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAlzRoJEACgkQ9AfbAGHV z0BITAf7BOBab8SYDY+6gzStYBKkbNWYaDyVcK64UVTQh7T8KEaOCJbp/bMSbVlt 4072LdAvAeYydoD6Qi1Fj27y9qsraSm5pjNvUvl70eVFmE+AxFL2Sl24W5Vb6BjA LRsWP+QgtyDRUnwPXRX99Qo8cEg/u7dEC5M2cknFiZMfhgZtEkSuTcjfbAUURB5q 2sbALOQKdXN2/NtpkZad5dbxp/s51RFGe6dmfnP0PtJLHGNLkIpNv0ttG+yAd2wd JEOp6/jTezSY59vA/ZedefES1D/wRpUo7j/NzKHalcnUA6Jc03OgVVUj2OccvMED LW1FP8IKEZvwLwc3TnfeqiZb9o4Vfg== =E+OM -----END PGP SIGNATURE----- --eMorwZOVWopjgZeMHJ77chDJmeBPGtSji--