Re: [Qemu-devel] [PATCH v6 25/42] mirror: Deal with filters

[Max Reitz <mreitz@redhat.com>]

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On 31.08.19 11:57, Vladimir Sementsov-Ogievskiy wrote:
> 09.08.2019 19:13, Max Reitz wrote:
>> This includes some permission limiting (for example, we only need to
>> take the RESIZE permission for active commits where the base is smaller
>> than the top).
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   block/mirror.c | 117 ++++++++++++++++++++++++++++++++++++++-----------
>>   blockdev.c     |  47 +++++++++++++++++---
>>   2 files changed, 131 insertions(+), 33 deletions(-)
>>
>> diff --git a/block/mirror.c b/block/mirror.c
>> index 54bafdf176..6ddbfb9708 100644
>> --- a/block/mirror.c
>> +++ b/block/mirror.c
>> @@ -42,6 +42,7 @@ typedef struct MirrorBlockJob {
>>       BlockBackend *target;
>>       BlockDriverState *mirror_top_bs;
>>       BlockDriverState *base;
>> +    BlockDriverState *base_overlay;
>>   
>>       /* The name of the graph node to replace */
>>       char *replaces;
>> @@ -665,8 +666,10 @@ static int mirror_exit_common(Job *job)
>>                                &error_abort);
>>       if (!abort && s->backing_mode == MIRROR_SOURCE_BACKING_CHAIN) {
>>           BlockDriverState *backing = s->is_none_mode ? src : s->base;
>> -        if (backing_bs(target_bs) != backing) {
>> -            bdrv_set_backing_hd(target_bs, backing, &local_err);
>> +        BlockDriverState *unfiltered_target = bdrv_skip_rw_filters(target_bs);
>> +
>> +        if (bdrv_filtered_cow_bs(unfiltered_target) != backing) {
>> +            bdrv_set_backing_hd(unfiltered_target, backing, &local_err);
>>               if (local_err) {
>>                   error_report_err(local_err);
>>                   ret = -EPERM;
>> @@ -715,7 +718,7 @@ static int mirror_exit_common(Job *job)
>>        * valid.
>>        */
>>       block_job_remove_all_bdrv(bjob);
>> -    bdrv_replace_node(mirror_top_bs, backing_bs(mirror_top_bs), &error_abort);
>> +    bdrv_replace_node(mirror_top_bs, mirror_top_bs->backing->bs, &error_abort);
>>   
>>       /* 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 cleanup does
>> @@ -812,7 +815,8 @@ static int coroutine_fn mirror_dirty_init(MirrorBlockJob *s)
>>               return 0;
>>           }
>>   
>> -        ret = bdrv_is_allocated_above(bs, base, false, offset, bytes, &count);
>> +        ret = bdrv_is_allocated_above(bs, s->base_overlay, true, offset, bytes,
>> +                                      &count);
>>           if (ret < 0) {
>>               return ret;
>>           }
>> @@ -908,7 +912,7 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
>>       } else {
>>           s->target_cluster_size = BDRV_SECTOR_SIZE;
>>       }
>> -    if (backing_filename[0] && !target_bs->backing &&
>> +    if (backing_filename[0] && !bdrv_backing_chain_next(target_bs) &&
>>           s->granularity < s->target_cluster_size) {
>>           s->buf_size = MAX(s->buf_size, s->target_cluster_size);
>>           s->cow_bitmap = bitmap_new(length);
>> @@ -1088,8 +1092,9 @@ static void mirror_complete(Job *job, Error **errp)
>>       if (s->backing_mode == MIRROR_OPEN_BACKING_CHAIN) {
>>           int ret;
>>   
>> -        assert(!target->backing);
>> -        ret = bdrv_open_backing_file(target, NULL, "backing", errp);
>> +        assert(!bdrv_backing_chain_next(target));
> 
> Preexisting, but seems we may crash here, I don't see where it is checked before, to
> return error if there is some backing. And even if we do so, we don't prevent appearing
> of target backing during mirror operation.

The idea is that MIRROR_OPEN_BACKING_CHAIN is set only when using
drive-mirror with mode=existing.  In this case, we also set
BDRV_O_NO_BACKING for the target.

You’re right that a user could add a backing chain to the target during
the operation.  They really have to make an effort to shoot themselves
in the foot for this because the target must have an auto-generated node
name.

I suppose the best would be not to open the backing chain if the target
node already has a backing child?

>> +        ret = bdrv_open_backing_file(bdrv_skip_rw_filters(target), NULL,
>> +                                     "backing", errp);
>>           if (ret < 0) {
>>               return;
>>           }
>> @@ -1531,8 +1536,8 @@ static BlockJob *mirror_start_job(
>>       MirrorBlockJob *s;
>>       MirrorBDSOpaque *bs_opaque;
>>       BlockDriverState *mirror_top_bs;
>> -    bool target_graph_mod;
>>       bool target_is_backing;
>> +    uint64_t target_perms, target_shared_perms;
>>       Error *local_err = NULL;
>>       int ret;
>>   
>> @@ -1551,7 +1556,7 @@ static BlockJob *mirror_start_job(
>>           buf_size = DEFAULT_MIRROR_BUF_SIZE;
>>       }
>>   
>> -    if (bs == target) {
>> +    if (bdrv_skip_rw_filters(bs) == bdrv_skip_rw_filters(target)) {
>>           error_setg(errp, "Can't mirror node into itself");
>>           return NULL;
>>       }
>> @@ -1615,15 +1620,50 @@ static BlockJob *mirror_start_job(
>>        * In the case of active commit, things look a bit different, though,
>>        * because the target is an already populated backing file in active use.
>>        * We can allow anything except resize there.*/
>> +
>> +    target_perms = BLK_PERM_WRITE;
>> +    target_shared_perms = BLK_PERM_WRITE_UNCHANGED;
>> +
>>       target_is_backing = bdrv_chain_contains(bs, target);
>> -    target_graph_mod = (backing_mode != MIRROR_LEAVE_BACKING_CHAIN);
>> +    if (target_is_backing) {
>> +        int64_t bs_size, target_size;
> 
> <empty after definitions>

Is that part of any of our guidelines? :-)

Sure, will add.

>> +        bs_size = bdrv_getlength(bs);
>> +        if (bs_size < 0) {
>> +            error_setg_errno(errp, -bs_size,
>> +                             "Could not inquire top image size");
>> +            goto fail;
>> +        }
>> +
>> +        target_size = bdrv_getlength(target);
>> +        if (target_size < 0) {
>> +            error_setg_errno(errp, -target_size,
>> +                             "Could not inquire base image size");
>> +            goto fail;
>> +        }
>> +
>> +        if (target_size < bs_size) {
>> +            target_perms |= BLK_PERM_RESIZE;
>> +        }
>> +
>> +        target_shared_perms |= BLK_PERM_CONSISTENT_READ
>> +                            |  BLK_PERM_WRITE
>> +                            |  BLK_PERM_GRAPH_MOD;
>> +    } else if (bdrv_chain_contains(bs, bdrv_skip_rw_filters(target))) {
>> +        /*
>> +         * We may want to allow this in the future, but it would
>> +         * require taking some extra care.
>> +         */
>> +        error_setg(errp, "Cannot mirror to a filter on top of a node in the "
>> +                   "source's backing chain");
>> +        goto fail;
>> +    }
>> +
>> +    if (backing_mode != MIRROR_LEAVE_BACKING_CHAIN) {
>> +        target_perms |= BLK_PERM_GRAPH_MOD;
>> +    }
>> +
>>       s->target = blk_new(s->common.job.aio_context,
>> -                        BLK_PERM_WRITE | BLK_PERM_RESIZE |
>> -                        (target_graph_mod ? BLK_PERM_GRAPH_MOD : 0),
>> -                        BLK_PERM_WRITE_UNCHANGED |
>> -                        (target_is_backing ? BLK_PERM_CONSISTENT_READ |
>> -                                             BLK_PERM_WRITE |
>> -                                             BLK_PERM_GRAPH_MOD : 0));
>> +                        target_perms, target_shared_perms);
>>       ret = blk_insert_bs(s->target, target, errp);
>>       if (ret < 0) {
>>           goto fail;
>> @@ -1647,6 +1687,7 @@ static BlockJob *mirror_start_job(
>>       s->backing_mode = backing_mode;
>>       s->copy_mode = copy_mode;
>>       s->base = base;
>> +    s->base_overlay = bdrv_find_overlay(bs, base);
>>       s->granularity = granularity;
>>       s->buf_size = ROUND_UP(buf_size, granularity);
>>       s->unmap = unmap;
>> @@ -1693,15 +1734,39 @@ static BlockJob *mirror_start_job(
>>       /* In commit_active_start() all intermediate nodes disappear, so
>>        * any jobs in them must be blocked */
>>       if (target_is_backing) {
>> -        BlockDriverState *iter;
>> -        for (iter = backing_bs(bs); iter != target; iter = backing_bs(iter)) {
>> -            /* XXX BLK_PERM_WRITE needs to be allowed so we don't block
>> -             * ourselves at s->base (if writes are blocked for a node, they are
>> -             * also blocked for its backing file). The other options would be a
>> -             * second filter driver above s->base (== target). */
>> +        BlockDriverState *iter, *filtered_target;
>> +        uint64_t iter_shared_perms;
>> +
>> +        /*
>> +         * The topmost node with
>> +         * bdrv_skip_rw_filters(filtered_target) == bdrv_skip_rw_filters(target)
>> +         */
>> +        filtered_target = bdrv_filtered_cow_bs(bdrv_find_overlay(bs, target));
>> +
>> +        assert(bdrv_skip_rw_filters(filtered_target) ==
>> +               bdrv_skip_rw_filters(target));
>> +
>> +        /*
>> +         * XXX BLK_PERM_WRITE needs to be allowed so we don't block
>> +         * ourselves at s->base (if writes are blocked for a node, they are
>> +         * also blocked for its backing file). The other options would be a
>> +         * second filter driver above s->base (== target).
>> +         */
>> +        iter_shared_perms = BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE;
>> +
>> +        for (iter = bdrv_filtered_bs(bs); iter != target;
>> +             iter = bdrv_filtered_bs(iter))
>> +        {
>> +            if (iter == filtered_target) {
>> +                /*
>> +                 * From here on, all nodes are filters on the base.
>> +                 * This allows us to share BLK_PERM_CONSISTENT_READ.
> 
> I'd prefere to add something like: "because we share it on target (see target BlockBackend creation
> and corresponding comment above)".

I’d rather not refer to other comments in case they change…  Maybe just
“This allows us to share BLK_PERM_CONSISTENT_READ, as we do on the
target.”?  I think if someone is interested, they will scan the file for
what permissions are shared on the target anyway.

>> +                 */
>> +                iter_shared_perms |= BLK_PERM_CONSISTENT_READ;
>> +            }
>> +
>>               ret = block_job_add_bdrv(&s->common, "intermediate node", iter, 0,
>> -                                     BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE,
>> -                                     errp);
>> +                                     iter_shared_perms, errp);
>>               if (ret < 0) {
>>                   goto fail;
>>               }
>> @@ -1737,7 +1802,7 @@ fail:
>>       bs_opaque->stop = true;
>>       bdrv_child_refresh_perms(mirror_top_bs, mirror_top_bs->backing,
>>                                &error_abort);
>> -    bdrv_replace_node(mirror_top_bs, backing_bs(mirror_top_bs), &error_abort);
>> +    bdrv_replace_node(mirror_top_bs, mirror_top_bs->backing->bs, &error_abort);
>>   
>>       bdrv_unref(mirror_top_bs);
>>   
>> @@ -1764,7 +1829,7 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
>>           return;
>>       }
>>       is_none_mode = mode == MIRROR_SYNC_MODE_NONE;
>> -    base = mode == MIRROR_SYNC_MODE_TOP ? backing_bs(bs) : NULL;
>> +    base = mode == MIRROR_SYNC_MODE_TOP ? bdrv_backing_chain_next(bs) : NULL;
>>       mirror_start_job(job_id, bs, creation_flags, target, replaces,
>>                        speed, granularity, buf_size, backing_mode,
>>                        on_source_error, on_target_error, unmap, NULL, NULL,
>> diff --git a/blockdev.c b/blockdev.c
>> index c540802127..c451f553f7 100644
> 
> 
> block/mirroc.c is OK for me. Continue with blockdev.c...
> 
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -3851,7 +3851,7 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>>           return;
>>       }
>>   
>> -    if (!bs->backing && sync == MIRROR_SYNC_MODE_TOP) {
>> +    if (!bdrv_backing_chain_next(bs) && sync == MIRROR_SYNC_MODE_TOP) {
>>           sync = MIRROR_SYNC_MODE_FULL;
>>       }
>>   
>> @@ -3900,7 +3900,7 @@ static void blockdev_mirror_common(const char *job_id, BlockDriverState *bs,
>>   
>>   void qmp_drive_mirror(DriveMirror *arg, Error **errp)
>>   {
>> -    BlockDriverState *bs;
>> +    BlockDriverState *bs, *unfiltered_bs;
>>       BlockDriverState *source, *target_bs;
>>       AioContext *aio_context;
>>       BlockMirrorBackingMode backing_mode;
>> @@ -3909,6 +3909,7 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
>>       int flags;
>>       int64_t size;
>>       const char *format = arg->format;
>> +    const char *replaces_node_name = NULL;
>>       int ret;
>>   
>>       bs = qmp_get_root_bs(arg->device, errp);
>> @@ -3921,6 +3922,16 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
>>           return;
>>       }
>>   
>> +    /*
>> +     * If the user has not instructed us otherwise, we should let the
>> +     * block job run from @bs (thus taking into account all filters on
>> +     * it) but replace @unfiltered_bs when it finishes (thus not
>> +     * removing those filters).
>> +     * (And if there are any explicit filters, we should assume the
>> +     *  user knows how to use the @replaces option.)
>> +     */
>> +    unfiltered_bs = bdrv_skip_implicit_filters(bs);
>> +
>>       aio_context = bdrv_get_aio_context(bs);
>>       aio_context_acquire(aio_context);
>>   
>> @@ -3934,8 +3945,14 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
>>       }
>>   
>>       flags = bs->open_flags | BDRV_O_RDWR;
>> -    source = backing_bs(bs);
>> +    source = bdrv_filtered_cow_bs(unfiltered_bs);
>>       if (!source && arg->sync == MIRROR_SYNC_MODE_TOP) {
> 
> 
> Hmm, you handle this case a bit differently here and in blockdev_mirror_common..
> Can we handle it only in blockdev_mirror_common, to be consistent with qmp_blockdev_mirror?

What exactly do you mean?  The difference between skipping all filters
and just skipping implicit filters?  Hm.

First, the check in blockdev_mirror_common() should actually be
unnecessary.  In qmp_{blockdev,drive}_mirror(), we do nearly the same
check anyway (and then force sync=full if there is no backing file).  So
if all three functions did the same check, we wouldn’t need it in
blockdev_mirror_common().

Second, let’s look at the difference in an example: One where
blockdev_mirror_common() would not decide to enforce mode=full, but
qmp_{blockdev,drive}_mirror() would.
This happens when @bs is an explicit filter over some overlay with a
backing file, e.g.:

throttle --file--> qcow2 --backing--> raw

It’s correct to run the mirror job from the throttle node; but @source
should be bdrv_backing_chain_next() so it will point to the raw node.
Currently, it is NULL (because the throttle node does not have a COW child).

But then again, I’ve made qmp_{blockdev,drive}_mirror() throw an error
in such a case:

>> +        if (bdrv_filtered_bs(unfiltered_bs)) {
>> +            /* @unfiltered_bs is an explicit filter */
>> +            error_setg(errp, "Cannot perform sync=top mirror through an "
>> +                       "explicitly added filter node on the source");
>> +            goto out;
>> +        }

So it isn’t really a problem.  Still, does the error make sense?  Should
we just allow that case by letting source be
bdrv_filtered_cow_bs(bdrv_skip_rw_filters(bs))?

(BTW, I just noticed that @base seems to be pretty much unused in
block/mirror.c.  It only really uses @base_overlay now.  So I suppose it
makes sense to remove it in v7.)

>>           arg->sync = MIRROR_SYNC_MODE_FULL;
>>       }
>>       if (arg->sync == MIRROR_SYNC_MODE_NONE) {
>> @@ -3954,6 +3971,9 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
>>                                " named node of the graph");
>>               goto out;
>>           }
>> +        replaces_node_name = arg->replaces;
>> +    } else if (unfiltered_bs != bs) {
>> +        replaces_node_name = unfiltered_bs->node_name;
>>       }
>>   
>>       if (arg->mode == NEW_IMAGE_MODE_ABSOLUTE_PATHS) {
>> @@ -3973,6 +3993,9 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
>>           bdrv_img_create(arg->target, format,
>>                           NULL, NULL, NULL, size, flags, false, &local_err);
>>       } else {
>> +        /* Implicit filters should not appear in the filename */
>> +        BlockDriverState *explicit_backing = bdrv_skip_implicit_filters(source);
>> +
>>           switch (arg->mode) {
>>           case NEW_IMAGE_MODE_EXISTING:
>>               break;
>> @@ -3980,8 +4003,8 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
>>               /* create new image with backing file */
>>               bdrv_refresh_filename(source);
>>               bdrv_img_create(arg->target, format,
>> -                            source->filename,
>> -                            source->drv->format_name,
>> +                            explicit_backing->filename,
>> +                            explicit_backing->drv->format_name,
>>                               NULL, size, flags, false, &local_err);
>>               break;
>>           default:
>> @@ -4017,7 +4040,7 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
>>       }
>>   
>>       blockdev_mirror_common(arg->has_job_id ? arg->job_id : NULL, bs, target_bs,
>> -                           arg->has_replaces, arg->replaces, arg->sync,
>> +                           !!replaces_node_name, replaces_node_name, arg->sync,
>>                              backing_mode, arg->has_speed, arg->speed,
>>                              arg->has_granularity, arg->granularity,
>>                              arg->has_buf_size, arg->buf_size,
>> @@ -4053,7 +4076,7 @@ void qmp_blockdev_mirror(bool has_job_id, const char *job_id,
>>                            bool has_auto_dismiss, bool auto_dismiss,
>>                            Error **errp)
>>   {
>> -    BlockDriverState *bs;
>> +    BlockDriverState *bs, *unfiltered_bs;
>>       BlockDriverState *target_bs;
>>       AioContext *aio_context;
>>       BlockMirrorBackingMode backing_mode = MIRROR_LEAVE_BACKING_CHAIN;
>> @@ -4065,6 +4088,16 @@ void qmp_blockdev_mirror(bool has_job_id, const char *job_id,
>>           return;
>>       }
>>   
>> +    /*
>> +     * Same as in qmp_drive_mirror():
> 
> Then, may be better do it in blockdev_mirror_common ?

Hm, maybe.  Should we decide to let @source be
bdrv_filtered_cow_bs(bdrv_skip_rw_filters(bs)) in qmp_drive_mirror(), I
don’t think we need @unfiltered_bs there to determine @source.

Max

>> We want to run the job from @bs,
>> +     * but we want to replace @unfiltered_bs on completion.
>> +     */
>> +    unfiltered_bs = bdrv_skip_implicit_filters(bs);
>> +    if (!has_replaces && unfiltered_bs != bs) {
>> +        replaces = unfiltered_bs->node_name;
>> +        has_replaces = true;
>> +    }
>> +
>>       target_bs = bdrv_lookup_bs(target, target, errp);
>>       if (!target_bs) {
>>           return;
>>
> 
> 



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