Re: [Qemu-devel] [PATCH v10 3/4] block: Add replaces argument to drive-mirror

["Benoît Canet" <benoit.canet@irqsave.net>]

The Friday 27 Jun 2014 à 13:57:02 (+0200), Kevin Wolf wrote :
> Am 16.06.2014 um 12:00 hat Benoît Canet geschrieben:
> > drive-mirror will bdrv_swap the new BDS named node-name with the one
> > pointed by replaces when the mirroring is finished.
> > 
> > Signed-off-by: Benoit Canet <benoit@irqsave.net>
> > Reviewed-by: Max Reitz <mreitz@redhat.com>
> > ---
> >  block.c                   | 17 ++++++++++++++
> >  block/mirror.c            | 60 +++++++++++++++++++++++++++++++++++++----------
> >  blockdev.c                | 30 +++++++++++++++++++++++-
> >  hmp.c                     |  2 +-
> >  include/block/block.h     |  4 ++++
> >  include/block/block_int.h |  3 +++
> >  qapi/block-core.json      |  6 ++++-
> >  qmp-commands.hx           |  4 +++-
> >  8 files changed, 109 insertions(+), 17 deletions(-)
> > 
> > diff --git a/block.c b/block.c
> > index 17f763d..318f1e6 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -5795,3 +5795,20 @@ bool bdrv_is_first_non_filter(BlockDriverState *candidate)
> >  
> >      return false;
> >  }
> > +
> > +BlockDriverState *check_to_replace_node(const char *node_name, Error **errp)
> > +{
> > +    BlockDriverState *to_replace_bs = bdrv_find_node(node_name);
> > +    if (!to_replace_bs) {
> > +        error_setg(errp, "Node name '%s' not found",
> > +                   node_name);
> 
> Unnecessary line break.
> 
> > +        return NULL;
> > +    }
> > +
> > +    if (bdrv_op_is_blocked(to_replace_bs, BLOCK_OP_TYPE_REPLACE, errp)) {
> > +        return NULL;
> > +    }
> > +
> > +    return to_replace_bs;
> > +}
> > +
> 
> Empty line before EOF.
> 
> > diff --git a/block/mirror.c b/block/mirror.c
> > index 94c8661..151167e 100644
> > --- a/block/mirror.c
> > +++ b/block/mirror.c
> > @@ -32,6 +32,12 @@ typedef struct MirrorBlockJob {
> >      RateLimit limit;
> >      BlockDriverState *target;
> >      BlockDriverState *base;
> > +    /* The name of the graph node to replace */
> > +    char *replaces;
> > +    /* The BDS to replace */
> > +    BlockDriverState *to_replace;
> > +    /* Used to block operations on the drive-mirror-replace target */
> > +    Error *replace_blocker;
> >      bool is_none_mode;
> >      BlockdevOnError on_source_error, on_target_error;
> >      bool synced;
> > @@ -490,10 +496,14 @@ immediate_exit:
> >      bdrv_release_dirty_bitmap(bs, s->dirty_bitmap);
> >      bdrv_iostatus_disable(s->target);
> >      if (s->should_complete && ret == 0) {
> > -        if (bdrv_get_flags(s->target) != bdrv_get_flags(s->common.bs)) {
> > -            bdrv_reopen(s->target, bdrv_get_flags(s->common.bs), NULL);
> > +        BlockDriverState *to_replace = s->common.bs;
> > +        if (s->to_replace) {
> > +            to_replace = s->to_replace;
> >          }
> > -        bdrv_swap(s->target, s->common.bs);
> > +        if (bdrv_get_flags(s->target) != bdrv_get_flags(to_replace)) {
> > +            bdrv_reopen(s->target, bdrv_get_flags(to_replace), NULL);
> > +        }
> > +        bdrv_swap(s->target, to_replace);
> >          if (s->common.driver->job_type == BLOCK_JOB_TYPE_COMMIT) {
> >              /* drop the bs loop chain formed by the swap: break the loop then
> >               * trigger the unref from the top one */
> > @@ -502,6 +512,12 @@ immediate_exit:
> >              bdrv_unref(p);
> >          }
> >      }
> > +    if (s->to_replace) {
> > +        bdrv_op_unblock_all(s->to_replace, s->replace_blocker);
> > +        error_free(s->replace_blocker);
> > +        bdrv_unref(s->to_replace);
> > +    }
> > +    g_free(s->replaces);
> >      bdrv_unref(s->target);
> >      block_job_completed(&s->common, ret);
> >  }
> > @@ -540,6 +556,20 @@ static void mirror_complete(BlockJob *job, Error **errp)
> >          return;
> >      }
> >  
> > +    /* check the target bs is not blocked and block all operations on it */
> > +    if (s->replaces) {
> > +        s->to_replace = check_to_replace_node(s->replaces, errp);
> > +
> 
> This empty line looks unusual.
> 
> > +        if (!s->to_replace) {
> > +            return;
> > +        }
> 
> So here is the thing that I really wanted to comment on. In the case of
> a REPLACE blocker being set, this is a silent failure. The completion
> command will return success, but s->should_complete won't actually be
> set, so the completion doesn't happen. The only thing that actually
> happens is the bdrv_open_backing_file(s->target) (which looks somewhat
> questionable, too...)
> 
> Now I would expect that the REPLACE blocker is actually set for any
> backing file, because that is what bdrv_set_backing_hd() does. For
> quorum it does work as expected because quorum children don't get any
> backing_blocker (we need to check whether they should get something
> similar from the quorum BDS), so this is probably why it escaped your
> testing. We'll need a test case that tries replacing some ordinary
> backing file.
> 
> Now I think the (accidental?) restriction to only replacing quorum nodes
> actually makes this patch pretty safe, so maybe it would be nice to keep
> this behaviour; but we need to fix it to not fail silently but return an
> explicit error.
> 
> > +        error_setg(&s->replace_blocker,
> > +                   "block device is in use by block-job-complete");
> > +        bdrv_op_block_all(s->to_replace, s->replace_blocker);
> > +        bdrv_ref(s->to_replace);
> > +    }
> > +
> >      s->should_complete = true;
> >      block_job_resume(job);
> >  }
> > @@ -562,14 +592,15 @@ static const BlockJobDriver commit_active_job_driver = {
> >  };
> >  
> >  static void mirror_start_job(BlockDriverState *bs, BlockDriverState *target,
> > -                            int64_t speed, int64_t granularity,
> > -                            int64_t buf_size,
> > -                            BlockdevOnError on_source_error,
> > -                            BlockdevOnError on_target_error,
> > -                            BlockDriverCompletionFunc *cb,
> > -                            void *opaque, Error **errp,
> > -                            const BlockJobDriver *driver,
> > -                            bool is_none_mode, BlockDriverState *base)
> > +                             const char *replaces,
> > +                             int64_t speed, int64_t granularity,
> > +                             int64_t buf_size,
> > +                             BlockdevOnError on_source_error,
> > +                             BlockdevOnError on_target_error,
> > +                             BlockDriverCompletionFunc *cb,
> > +                             void *opaque, Error **errp,
> > +                             const BlockJobDriver *driver,
> > +                             bool is_none_mode, BlockDriverState *base)
> >  {
> >      MirrorBlockJob *s;
> >  
> > @@ -600,6 +631,7 @@ static void mirror_start_job(BlockDriverState *bs, BlockDriverState *target,
> >          return;
> >      }
> >  
> > +    s->replaces = g_strdup(replaces);
> >      s->on_source_error = on_source_error;
> >      s->on_target_error = on_target_error;
> >      s->target = target;
> 
> One design question that isn't quite clear to me yet is why you resolve
> the device name only in mirror_complete() and not here. This means that
> the drive-mirror QMP command can refer to one BDS with node-name foo,
> which then gets removed and another BDS with node-name foo is added, and
> then it would refer to the new BDS on completion time.
> 
> I would find it less surprising if we took a reference to the old BDS
> here so that you can't remove it. Perhaps setting the replace_blocker
> here already would be safer, too.

I have done late binding because this allow the code to block the to replace
BDS late. If I get a reference to it early the code must block it early.

> 
> > @@ -621,6 +653,7 @@ static void mirror_start_job(BlockDriverState *bs, BlockDriverState *target,
> >  }
> >  
> >  void mirror_start(BlockDriverState *bs, BlockDriverState *target,
> > +                  const char *replaces,
> >                    int64_t speed, int64_t granularity, int64_t buf_size,
> >                    MirrorSyncMode mode, BlockdevOnError on_source_error,
> >                    BlockdevOnError on_target_error,
> > @@ -632,7 +665,8 @@ void mirror_start(BlockDriverState *bs, BlockDriverState *target,
> >  
> >      is_none_mode = mode == MIRROR_SYNC_MODE_NONE;
> >      base = mode == MIRROR_SYNC_MODE_TOP ? bs->backing_hd : NULL;
> > -    mirror_start_job(bs, target, speed, granularity, buf_size,
> > +    mirror_start_job(bs, target, replaces,
> > +                     speed, granularity, buf_size,
> >                       on_source_error, on_target_error, cb, opaque, errp,
> >                       &mirror_job_driver, is_none_mode, base);
> >  }
> > @@ -680,7 +714,7 @@ void commit_active_start(BlockDriverState *bs, BlockDriverState *base,
> >      }
> >  
> >      bdrv_ref(base);
> > -    mirror_start_job(bs, base, speed, 0, 0,
> > +    mirror_start_job(bs, base, NULL, speed, 0, 0,
> >                       on_error, on_error, cb, opaque, &local_err,
> >                       &commit_active_job_driver, false, base);
> >      if (local_err) {
> > diff --git a/blockdev.c b/blockdev.c
> > index 06b14f2..237a548 100644
> > --- a/blockdev.c
> > +++ b/blockdev.c
> > @@ -2107,6 +2107,7 @@ BlockDeviceInfoList *qmp_query_named_block_nodes(Error **errp)
> >  void qmp_drive_mirror(const char *device, const char *target,
> >                        bool has_format, const char *format,
> >                        bool has_node_name, const char *node_name,
> > +                      bool has_replaces, const char *replaces,
> >                        enum MirrorSyncMode sync,
> >                        bool has_mode, enum NewImageMode mode,
> >                        bool has_speed, int64_t speed,
> > @@ -2194,6 +2195,28 @@ void qmp_drive_mirror(const char *device, const char *target,
> >          return;
> >      }
> >  
> > +    if (has_replaces) {
> > +        BlockDriverState *to_replace_bs;
> > +
> > +        if (!has_node_name) {
> > +            error_setg(errp, "a node-name must be provided when replacing a"
> > +                             " named node of the graph");
> > +            return;
> > +        }
> > +
> > +        to_replace_bs = check_to_replace_node(replaces, errp);
> > +
> > +        if (!to_replace_bs) {
> > +            return;
> > +        }
> > +
> > +        if (size != bdrv_getlength(to_replace_bs)) {
> > +            error_setg(errp, "cannot replace image with a mirror image of "
> > +                             "different size");
> > +            return;
> > +        }
> 
> We may want to loosen some of these restrictions later, but it's good to
> start with more restrictions if in doubt.
> 
> > +    }
> > +
> >      if ((sync == MIRROR_SYNC_MODE_FULL || !source)
> >          && mode != NEW_IMAGE_MODE_EXISTING)
> >      {
> 
> Kevin