Re: [PATCH v3 4/7] migration/block-dirty-bitmap: fix bitmaps pre-blockdev migration during mirror job

[Eric Blake <eblake@redhat.com>]

On 5/15/20 7:40 AM, Vladimir Sementsov-Ogievskiy wrote:
> Important thing for bitmap migration is to select destination block
> node to obtain the migrated bitmap.
> 
> Prepatch, on source we use bdrv_get_device_or_node_name() to identify
> the node, and on target we do bdrv_lookup_bs.
> bdrv_get_device_or_node_name() returns blk name only for direct
> children of blk. So, bitmaps of direct children of blks are migrated by
> blk name and others - by node name.
> 
> Old libvirt is unprepared to bitmap migration by node-name,
> node-names are mostly auto-generated. So actually only migration by blk
> name works for it.
> 
> Newer libvirt will use new interface (which will be added soon) to
> specify node-mapping for bitmaps migration explicitly. Still, let's
> improve the current behavior a bit.
> 
> Now, consider classic libvirt migrations assisted by mirror block job:
> mirror block job inserts filter, so our source is not a direct child of
> blk, and bitmaps are migrated by node-names. And this just don't work

either "won't" or "doesn't"

> with auto-generated node names

trailing '.'

> 
> Let's fix it by allowing use blk-name even if some implicit filters are
> inserted.

s/allowing use/using/

> 
> Note2: we, of course, can't skip filters and use blk name to migrate
> bitmaps in filtered node by blk name for this blk if these filters have
> named bitmaps which should be migrated.
> 
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1652424
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   migration/block-dirty-bitmap.c | 39 +++++++++++++++++++++++++++++++++-
>   1 file changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c
> index 7e93718086..5d3a7d2b07 100644
> --- a/migration/block-dirty-bitmap.c
> +++ b/migration/block-dirty-bitmap.c
> @@ -319,14 +319,48 @@ static int init_dirty_bitmap_migration(void)
>   {
>       BlockDriverState *bs;
>       DirtyBitmapMigBitmapState *dbms;
> +    GHashTable *handled_by_blk = g_hash_table_new(NULL, NULL);
> +    BlockBackend *blk;
>   
>       dirty_bitmap_mig_state.bulk_completed = false;
>       dirty_bitmap_mig_state.prev_bs = NULL;
>       dirty_bitmap_mig_state.prev_bitmap = NULL;
>       dirty_bitmap_mig_state.no_bitmaps = false;
>   
> +    /*
> +     * Use blockdevice name for direct (or filtered) children of named block
> +     * backends.
> +     */
> +    for (blk = blk_next(NULL); blk; blk = blk_next(blk)) {
> +        const char *name = blk_name(blk);
> +
> +        if (!name || strcmp(name, "") == 0) {
> +            continue;
> +        }
> +
> +        bs = blk_bs(blk);
> +
> +        /* Skip filters without bitmaos */
> +        while (bs && bs->drv && bs->drv->is_filter &&
> +               !bdrv_has_named_bitmaps(bs))
> +        {
> +            bs = bs->backing->bs ?: bs->file->bs;

Is this correct, or should it be:

bs = bs->backing ? bs->backing->bs : bs->file->bs;

Otherwise looks reasonable, but I'm hesitant to include it in today's 
bitmap pull request in order to give it more review/testing time.  It 
should be ready for a pull request next week, though.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org