On Thu, Oct 02, 2014 at 06:52:52PM +1000, Alexey Kardashevskiy wrote:
> When migrated using libvirt with "--copy-storage-all", at the end of
> migration there is race between NBD mirroring task trying to do flush
> and migration completion, both end up invalidating cache. Since qcow2
> driver does not handle this situation very well, random crashes happen.
>
> This disables the BDRV_O_INCOMING flag for the block device being migrated
> and restores it when NBD task is done.
>
> Signed-off-by: Alexey Kardashevskiy <a...@ozlabs.ru>
> ---
>
>
> The commit log is not full and most likely incorrect as well
> as the patch :) Please, help. Thanks!
>
> The patch seems to fix the initial problem though.
>
>
> btw is there any easy way to migrate one QEMU to another
> using NBD (i.e. not using "migrate -b") and not using libvirt?
> What would the command line be? Debugging with libvirt is real
> pain :(
>
>
> ---
> block.c | 17 ++++-------------
> migration.c | 1 -
> nbd.c | 11 +++++++++++
> 3 files changed, 15 insertions(+), 14 deletions(-)
>
> diff --git a/block.c b/block.c
> index c5a251c..ed72e0a 100644
> --- a/block.c
> +++ b/block.c
> @@ -5073,6 +5073,10 @@ void bdrv_invalidate_cache_all(Error **errp)
> QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> AioContext *aio_context = bdrv_get_aio_context(bs);
>
> + if (!(bs->open_flags & BDRV_O_INCOMING)) {
> + continue;
> + }
> +
We shouldn't touch bs before acquiring the AioContext. Acquiring the
AioContext is basically the "lock" for the BDS.
It needs to be moved...
> aio_context_acquire(aio_context);
...in here.
> bdrv_invalidate_cache(bs, &local_err);
> aio_context_release(aio_context);
> @@ -5083,19 +5087,6 @@ void bdrv_invalidate_cache_all(Error **errp)
> }
> }
>
> -void bdrv_clear_incoming_migration_all(void)
> -{
> - BlockDriverState *bs;
> -
> - QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> - AioContext *aio_context = bdrv_get_aio_context(bs);
> -
> - aio_context_acquire(aio_context);
> - bs->open_flags = bs->open_flags & ~(BDRV_O_INCOMING);
> - aio_context_release(aio_context);
> - }
> -}
> -
> int bdrv_flush(BlockDriverState *bs)
> {
> Coroutine *co;
> diff --git a/migration.c b/migration.c
> index 8d675b3..c49a05a 100644
> --- a/migration.c
> +++ b/migration.c
> @@ -103,7 +103,6 @@ static void process_incoming_migration_co(void *opaque)
> }
> qemu_announce_self();
>
> - bdrv_clear_incoming_migration_all();
> /* Make sure all file formats flush their mutable metadata */
> bdrv_invalidate_cache_all(&local_err);
BDRV_O_INCOMING needs to be cleared, otherwise the block drivers will
think that incoming migration is still taking place and treat the file
as effectively read-only during open.
On IRC I suggested doing away with the bdrv_invalidate_cache_all() name
since it no longer calls bdrv_invalidate_cache() on all BDSes.
Combine both clearing BDRV_O_INCOMING and calling
bdrv_invalidate_cache() if BDRV_O_INCOMING was previously set into one
function - you could reuse bdrv_clear_incoming_migration_all() for that.
> if (local_err) {
> diff --git a/nbd.c b/nbd.c
> index e9b539b..7b479c0 100644
> --- a/nbd.c
> +++ b/nbd.c
> @@ -106,6 +106,7 @@ struct NBDExport {
> off_t dev_offset;
> off_t size;
> uint32_t nbdflags;
> + bool restore_incoming;
Not needed, it does not make sense to restore BDRV_O_INCOMING because
once we've written to a file it cannot be in use by another host at the
same time.
> QTAILQ_HEAD(, NBDClient) clients;
> QTAILQ_ENTRY(NBDExport) next;
>
> @@ -972,6 +973,13 @@ NBDExport *nbd_export_new(BlockDriverState *bs, off_t
> dev_offset,
> exp->ctx = bdrv_get_aio_context(bs);
> bdrv_ref(bs);
> bdrv_add_aio_context_notifier(bs, bs_aio_attached, bs_aio_detach, exp);
> +
> + if (bs->open_flags & BDRV_O_INCOMING) {
I think the flag has to be cleared before calling
bdrv_invalidate_cache() because the .bdrv_open() function looks at the
flag.
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