Re: [PATCH v4 5/5] qcow2: repair a dirty image when it becomes writable

[Markus Armbruster <armbru@redhat.com>]

"Denis V. Lunev" <den@openvz.org> writes:

> From: Denis V. Lunev <den@openvz.org>
>
> A dirty image must be repaired before anything allocates a cluster in
> it. qcow2_do_open() does that, but only for a node that is writable
> from the start. A node opened read-only skips it, and nothing revisits
> the question once that node becomes writable, which block-commit does
> routinely: commit_active_start() and commit_start() reopen the base
> read-write for the duration of the job.
>
> With lazy refcounts the on-disk refcount block then still accounts for
> the metadata clusters only, so the allocator restarts at the front of
> the image and hands out clusters that L2 entries point at. Two guest
> offsets end up sharing one host cluster. Nothing fails, the corrupt bit
> stays clear, and a clean close clears the dirty bit, so no later open
> repairs the image either. The bit also stays set for the whole writable
> session, so a node which is merely writable says nothing.
>
> Refusing the reopen instead is simpler and keeps it atomic, but it
> leaves nowhere to go: the base belongs to a chain the VM has open, so
> the qemu-img check -r such an error would ask for cannot take the write
> lock it needs. The repair does the trick in most cases anyway.
>
> Do the repair in qcow2_reopen_commit_post(), the earliest point where
> the node is writable. An inactive node is skipped: bdrv_activate() calls
> qcow2_do_open() again through qcow2_co_invalidate_cache().
>
> commit_post cannot reject the reopen, so a failed repair takes the
> driver away from the node instead, which is what stops writes from
> aliasing live clusters. qcow2_signal_corruption() does that as well,
> but it also sends BLOCK_IMAGE_CORRUPTED and sets the corrupt bit,
> which qcow2_do_open() honours by refusing every later read-write open.
> The image is dirty and unrepaired, not corrupt, and qemu-img check -r
> still fixes it, so neither belongs here. Return the error and skip
> the bitmaps.
>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> Reviewed-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Hanna Reitz <hreitz@redhat.com>
> CC: Eric Blake <eblake@redhat.com>
> CC: Markus Armbruster <armbru@redhat.com>
> CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> Cc: qemu-stable@nongnu.org

[...]

> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 199efc1e00..940249a5e5 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -1852,6 +1852,9 @@
   ##
   # @change-backing-file:
   #
   # Change the backing file in the image file metadata.  This does not
   # cause QEMU to reopen the image file to reparse the backing filename
   # (it may, however, perform a reopen to change permissions from r/o ->
>  # r/w -> r/o, if needed).  The new backing file string is written into
>  # the image file metadata, and the QEMU internal strings are updated.
>  #
> +# A dirty qcow2 image is repaired during that reopen, which blocks
> +# other requests and can fail the command.

Pardon my ignorance: what makes a qcow2 image dirty?

Can you give me an idea of what other requests could be blocked?

Double-checking: "that reopen" is the one to change permissions,
i.e. the parenthesis above.  Correct?

> +#
>  # @image-node-name: The name of the block driver state node of the
>  #     image to modify.  The "device" argument is used to verify
>  #     "image-node-name" is in the chain described by "device".
> @@ -1891,6 +1894,9 @@
   ##
   # @block-commit:
   #
   # Live commit of data from overlay image nodes into backing nodes -
   # i.e., writes data between 'top' and 'base' into 'base'.
   #
   # If top == base, that is an error.  If top has no overlays on top of
   # it, or if it is in use by a writer, the job will not be completed by
   # itself.  The user needs to complete the job with the `job-complete`
   # command after getting the ready event.  (Since 2.0)
   #
   # If the base image is smaller than top, then the base image will be
   # resized to be the same size as top.  If top is smaller than the base
   # image, the base will not be truncated.  If you want the base image
>  # size to match the size of the smaller top, you can safely truncate
>  # it yourself once the commit operation successfully completes.
>  #
> +# The base is opened read-write.  A dirty qcow2 base is repaired
> +# first, which blocks other requests and can fail the command.
> +#
>  # @job-id: identifier for the newly-created block job.  If omitted,
>  #     the device name will be used.  (Since 2.7)
>  #
> @@ -2902,6 +2908,9 @@
   ##
   # @block-stream:
   #
   # Copy data from a backing file into a block device.

[...]

>  # On successful completion the image file is updated to drop the
>  # backing file and the `BLOCK_JOB_COMPLETED` event is emitted.
>  #
> +# The top image is opened read-write.  A dirty qcow2 image is repaired
> +# first, which blocks other requests and can fail the command.
> +#
>  # In case @device is a filter node, `block-stream` modifies the first
>  # non-filter overlay node below it to point to the new backing node
>  # instead of modifying @device itself.
> @@ -4989,6 +4998,16 @@
   ##
   # @blockdev-reopen:
   #
   # Reopens one or more block devices using the given set of options.
   # Any option not specified will be reset to its default value
   # regardless of its previous status.  If an option cannot be changed
   # or a particular driver does not support reopening then the command
   # will return an error.  All devices in the list are reopened in one
>  # transaction, so if one of them fails then the whole transaction is
>  # cancelled.
>  #
> +# An error is also returned when a device cannot be used once it has
> +# been reopened.  Such a reopen is not undone, so an error does not
> +# always mean that nothing has changed.

Could that be a problem?

> +#                                        A node the driver gave up on
> +# serves nothing at all: it keeps its image open, makes
> +# `query-named-block-nodes` fail for as long as it is in the graph,
> +# and `blockdev-del` removes it only once nothing refers to it.
> +#
> +# Reopening a dirty qcow2 image read-write repairs it first, which
> +# blocks other requests and can fail the command.
> +#
>  # The command receives a list of block devices to reopen.  For each
>  # one of them, the top-level @node-name option (from
>  # `BlockdevOptions`) must be specified and is used to select the block

These documentation updates suggest the patch affects commands
change-backing-file, block-commit, block-stream, and blockdev-reopen.
Is that correct?

Would it make sense to list them in the commit message?

[...]