If the top node's driver does not provide snapshot functionality and we want to fall back to a node down the chain, we need to snapshot all non-COW children. For simplicity's sake, just do not fall back if there is more than one such child.
bdrv_snapshot_goto() becomes a bit weird because we may have to redirect the actual child pointer, so it only works if the fallback child is bs->file or bs->backing (and then we have to find out which it is). Suggested-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> Signed-off-by: Max Reitz <mre...@redhat.com> --- block/snapshot.c | 100 +++++++++++++++++++++++++++++++++++++---------- 1 file changed, 79 insertions(+), 21 deletions(-) diff --git a/block/snapshot.c b/block/snapshot.c index f2f48f926a..35403c167f 100644 --- a/block/snapshot.c +++ b/block/snapshot.c @@ -146,6 +146,32 @@ bool bdrv_snapshot_find_by_id_and_name(BlockDriverState *bs, return ret; } +/** + * Return the child BDS to which we can fall back if the given BDS + * does not support snapshots. + * Return NULL if there is no BDS to (safely) fall back to. + */ +static BlockDriverState *bdrv_snapshot_fallback(BlockDriverState *bs) +{ + BlockDriverState *child_bs = NULL; + BdrvChild *child; + + QLIST_FOREACH(child, &bs->children, next) { + if (child == bdrv_filtered_cow_child(bs)) { + /* Ignore: COW children need not be included in snapshots */ + continue; + } + + if (child_bs) { + /* Cannot fall back to a single child if there are multiple */ + return NULL; + } + child_bs = child->bs; + } + + return child_bs; +} + int bdrv_can_snapshot(BlockDriverState *bs) { BlockDriver *drv = bs->drv; @@ -154,8 +180,9 @@ int bdrv_can_snapshot(BlockDriverState *bs) } if (!drv->bdrv_snapshot_create) { - if (bs->file != NULL) { - return bdrv_can_snapshot(bs->file->bs); + BlockDriverState *fallback_bs = bdrv_snapshot_fallback(bs); + if (fallback_bs) { + return bdrv_can_snapshot(fallback_bs); } return 0; } @@ -167,14 +194,15 @@ int bdrv_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info) { BlockDriver *drv = bs->drv; + BlockDriverState *fallback_bs = bdrv_snapshot_fallback(bs); if (!drv) { return -ENOMEDIUM; } if (drv->bdrv_snapshot_create) { return drv->bdrv_snapshot_create(bs, sn_info); } - if (bs->file) { - return bdrv_snapshot_create(bs->file->bs, sn_info); + if (fallback_bs) { + return bdrv_snapshot_create(fallback_bs, sn_info); } return -ENOTSUP; } @@ -184,6 +212,7 @@ int bdrv_snapshot_goto(BlockDriverState *bs, Error **errp) { BlockDriver *drv = bs->drv; + BlockDriverState *fallback_bs; int ret, open_ret; if (!drv) { @@ -204,39 +233,66 @@ int bdrv_snapshot_goto(BlockDriverState *bs, return ret; } - if (bs->file) { - BlockDriverState *file; - QDict *options = qdict_clone_shallow(bs->options); + fallback_bs = bdrv_snapshot_fallback(bs); + if (fallback_bs) { + QDict *options; QDict *file_options; Error *local_err = NULL; + bool is_backing_child; + BdrvChild **child_pointer; + + /* + * We need a pointer to the fallback child pointer, so let us + * see whether the child is referenced by a field in the BDS + * object. + */ + if (fallback_bs == bs->file->bs) { + is_backing_child = false; + child_pointer = &bs->file; + } else if (fallback_bs == bs->backing->bs) { + is_backing_child = true; + child_pointer = &bs->backing; + } else { + /* + * The fallback child is not referenced by a field in the + * BDS object. We cannot go on then. + */ + error_setg(errp, "Block driver does not support snapshots"); + return -ENOTSUP; + } + + options = qdict_clone_shallow(bs->options); - file = bs->file->bs; /* Prevent it from getting deleted when detached from bs */ - bdrv_ref(file); + bdrv_ref(fallback_bs); - qdict_extract_subqdict(options, &file_options, "file."); + qdict_extract_subqdict(options, &file_options, + is_backing_child ? "backing." : "file."); qobject_unref(file_options); - qdict_put_str(options, "file", bdrv_get_node_name(file)); + qdict_put_str(options, is_backing_child ? "backing" : "file", + bdrv_get_node_name(fallback_bs)); if (drv->bdrv_close) { drv->bdrv_close(bs); } - bdrv_unref_child(bs, bs->file); - bs->file = NULL; - ret = bdrv_snapshot_goto(file, snapshot_id, errp); + assert(fallback_bs == (*child_pointer)->bs); + bdrv_unref_child(bs, *child_pointer); + *child_pointer = NULL; + + ret = bdrv_snapshot_goto(fallback_bs, snapshot_id, errp); open_ret = drv->bdrv_open(bs, options, bs->open_flags, &local_err); qobject_unref(options); if (open_ret < 0) { - bdrv_unref(file); + bdrv_unref(fallback_bs); bs->drv = NULL; /* A bdrv_snapshot_goto() error takes precedence */ error_propagate(errp, local_err); return ret < 0 ? ret : open_ret; } - assert(bs->file->bs == file); - bdrv_unref(file); + assert(fallback_bs == (*child_pointer)->bs); + bdrv_unref(fallback_bs); return ret; } @@ -272,6 +328,7 @@ int bdrv_snapshot_delete(BlockDriverState *bs, Error **errp) { BlockDriver *drv = bs->drv; + BlockDriverState *fallback_bs = bdrv_snapshot_fallback(bs); int ret; if (!drv) { @@ -288,8 +345,8 @@ int bdrv_snapshot_delete(BlockDriverState *bs, if (drv->bdrv_snapshot_delete) { ret = drv->bdrv_snapshot_delete(bs, snapshot_id, name, errp); - } else if (bs->file) { - ret = bdrv_snapshot_delete(bs->file->bs, snapshot_id, name, errp); + } else if (fallback_bs) { + ret = bdrv_snapshot_delete(fallback_bs, snapshot_id, name, errp); } else { error_setg(errp, "Block format '%s' used by device '%s' " "does not support internal snapshot deletion", @@ -305,14 +362,15 @@ int bdrv_snapshot_list(BlockDriverState *bs, QEMUSnapshotInfo **psn_info) { BlockDriver *drv = bs->drv; + BlockDriverState *fallback_bs = bdrv_snapshot_fallback(bs); if (!drv) { return -ENOMEDIUM; } if (drv->bdrv_snapshot_list) { return drv->bdrv_snapshot_list(bs, psn_info); } - if (bs->file) { - return bdrv_snapshot_list(bs->file->bs, psn_info); + if (fallback_bs) { + return bdrv_snapshot_list(fallback_bs, psn_info); } return -ENOTSUP; } -- 2.21.0