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Serapheim Dimitropoulos
793c958f6f
= Motivation
We've noticed several zloop crashes within Delphix generated
due to the following sequence of events:
- A device gets expanded and new metaslabas are allocated for
it. These metaslabs go through `metaslab_init()` but haven't
gone through `metaslab_sync_done()` yet. This meas that the
only range tree that's actually set is the `ms_allocatable`.
All the others are NULL.
- A vdev_initialization is issues and `vdev_initialize_thread`
starts processing one of these new metaslabs of the expanded
vdev.
- As part of `vdev_initialize_calculate_progress()` we call
into `metaslab_load()` and `metaslab_load_impl()` which
in turn tries to dereference the metaslabs trees that
are still NULL and therefore we crash.
The same failure can come up from the `vdev_trim` code paths.
= This Patch
We considered the following solutions to deal with this issue:
[A] Add logic to `vdev_initialize/trim` to skip those new
metaslabs. We decided against this as it would be good
to avoid exposing this lower-level detail to higer-level
operations.
[B] Have `metaslab_load_impl()` return early for new metaslabs
and thus never touch those range_trees that are NULL at
that time. This seemed more of a work-around for the bug
and not a clear-cut solution.
[C] Refactor our logic so all metaslabs have their range_trees
created at the time of their creatin in `metaslab_init()`.
In this patch we decided to go with [C] because:
(1) It doesn't expose more metaslab details to higher level
operations such as vdev initialize and trim.
(2) The current behavior of creating the range trees lazily
in `metaslab_sync_done()` is unnecessarily complicated.
(3) Always initializing the metaslab range_trees makes other
parts of the codebase cleaner. For example, we used to
use `ms_freed` as the reference value for knowing whether
all the range_trees have been initialized. Now we no
longer need to do that check in most places (and in the
few that we do we use the `ms_new` boolean field now
which is more readable).
= Side Changes
Probably due to a mismerge we set `ms_loaded` to `B_TRUE` twice
in `metasloab_load_impl()`. In this patch we remove the extraneous
assignment.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #11737
OpenZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community. This repository contains the code for running OpenZFS on Linux and FreeBSD.
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