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Re: [Qemu-devel] [PATCH 13/16] block: Implement bdrv_append() without bd
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] [PATCH 13/16] block: Implement bdrv_append() without bdrv_swap() |
Date: |
Wed, 23 Sep 2015 18:36:50 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 |
On 17.09.2015 15:48, Kevin Wolf wrote:
> Remember all parent nodes and just change the pointers there instead of
> swapping the contents of the BlockDriverState.
>
> Handling of snapshot=on must be moved further down in bdrv_open()
> because *pbs (which is the bs pointer in the BlockBackend) must already
> be set before bdrv_append() is called. Otherwise bdrv_append() changes
> the BB's pointer to the temporary snapshot, but bdrv_open() overwrites
> it with the read-only original image.
>
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
> block.c | 109
> +++++++++++++++++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 81 insertions(+), 28 deletions(-)
>
> diff --git a/block.c b/block.c
> index c196f83..98fc17c 100644
> --- a/block.c
> +++ b/block.c
> @@ -1516,15 +1516,6 @@ static int bdrv_open_inherit(BlockDriverState **pbs,
> const char *filename,
>
> bdrv_refresh_filename(bs);
>
> - /* For snapshot=on, create a temporary qcow2 overlay. bs points to the
> - * temporary snapshot afterwards. */
> - if (snapshot_flags) {
> - ret = bdrv_append_temp_snapshot(bs, snapshot_flags, &local_err);
> - if (local_err) {
> - goto close_and_fail;
> - }
> - }
> -
> /* Check if any unknown options were used */
> if (options && (qdict_size(options) != 0)) {
> const QDictEntry *entry = qdict_first(options);
> @@ -1556,6 +1547,16 @@ static int bdrv_open_inherit(BlockDriverState **pbs,
> const char *filename,
>
> QDECREF(options);
> *pbs = bs;
> +
> + /* For snapshot=on, create a temporary qcow2 overlay. bs points to the
> + * temporary snapshot afterwards. */
> + if (snapshot_flags) {
> + ret = bdrv_append_temp_snapshot(bs, snapshot_flags, &local_err);
> + if (local_err) {
> + goto close_and_fail;
> + }
> + }
> +
> return 0;
>
> fail:
> @@ -1999,20 +2000,6 @@ static void bdrv_move_feature_fields(BlockDriverState
> *bs_dest,
>
> bs_dest->enable_write_cache = bs_src->enable_write_cache;
>
> - /* i/o throttled req */
> - bs_dest->throttle_state = bs_src->throttle_state,
> - bs_dest->io_limits_enabled = bs_src->io_limits_enabled;
> - bs_dest->pending_reqs[0] = bs_src->pending_reqs[0];
> - bs_dest->pending_reqs[1] = bs_src->pending_reqs[1];
> - bs_dest->throttled_reqs[0] = bs_src->throttled_reqs[0];
> - bs_dest->throttled_reqs[1] = bs_src->throttled_reqs[1];
> - memcpy(&bs_dest->round_robin,
> - &bs_src->round_robin,
> - sizeof(bs_dest->round_robin));
> - memcpy(&bs_dest->throttle_timers,
> - &bs_src->throttle_timers,
> - sizeof(ThrottleTimers));
> -
> /* r/w error */
> bs_dest->on_read_error = bs_src->on_read_error;
> bs_dest->on_write_error = bs_src->on_write_error;
> @@ -2026,10 +2013,25 @@ static void bdrv_move_feature_fields(BlockDriverState
> *bs_dest,
> }
>
> /* Fields that only need to be swapped if the contents of BDSes is swapped
> - * rather than pointers being changed in the parents. */
> + * rather than pointers being changed in the parents, and throttling fields
> + * because only bdrv_swap() messes with internals of throttling. */
> static void bdrv_move_reference_fields(BlockDriverState *bs_dest,
> BlockDriverState *bs_src)
> {
> + /* i/o throttled req */
> + bs_dest->throttle_state = bs_src->throttle_state,
> + bs_dest->io_limits_enabled = bs_src->io_limits_enabled;
> + bs_dest->pending_reqs[0] = bs_src->pending_reqs[0];
> + bs_dest->pending_reqs[1] = bs_src->pending_reqs[1];
> + bs_dest->throttled_reqs[0] = bs_src->throttled_reqs[0];
> + bs_dest->throttled_reqs[1] = bs_src->throttled_reqs[1];
> + memcpy(&bs_dest->round_robin,
> + &bs_src->round_robin,
> + sizeof(bs_dest->round_robin));
> + memcpy(&bs_dest->throttle_timers,
> + &bs_src->throttle_timers,
> + sizeof(ThrottleTimers));
> +
> /* reference count */
> bs_dest->refcnt = bs_src->refcnt;
>
> @@ -2155,6 +2157,42 @@ void bdrv_swap(BlockDriverState *bs_new,
> BlockDriverState *bs_old)
> bdrv_rebind(bs_old);
> }
>
> +static void change_parent_backing_link(BlockDriverState *from,
> + BlockDriverState *to)
> +{
> + BdrvChild *c, *next;
> +
> + QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
> + assert(c->role != &child_backing);
> + c->bs = to;
> + QLIST_REMOVE(c, next_parent);
> + QLIST_INSERT_HEAD(&to->parents, c, next_parent);
This drops a reference from the parent BDS to @from, and adds a new one
from the parent BDS to @to. However, this is not reflected here.
> + }
> + if (from->blk) {
> + blk_set_bs(from->blk, to);
> + if (!to->device_list.tqe_prev) {
> + QTAILQ_INSERT_BEFORE(from, to, device_list);
> + }
> + QTAILQ_REMOVE(&bdrv_states, from, device_list);
> + }
> +}
> +
> +static void swap_feature_fields(BlockDriverState *bs_top,
> + BlockDriverState *bs_new)
> +{
> + BlockDriverState tmp;
> +
> + bdrv_move_feature_fields(&tmp, bs_top);
> + bdrv_move_feature_fields(bs_top, bs_new);
> + bdrv_move_feature_fields(bs_new, &tmp);
> +
> + assert(!bs_new->io_limits_enabled);
> + if (bs_top->io_limits_enabled) {
> + bdrv_io_limits_enable(bs_new, throttle_group_get_name(bs_top));
> + bdrv_io_limits_disable(bs_top);
> + }
> +}
> +
> /*
> * Add new bs contents at the top of an image chain while the chain is
> * live, while keeping required fields on the top layer.
> @@ -2165,14 +2203,29 @@ void bdrv_swap(BlockDriverState *bs_new,
> BlockDriverState *bs_old)
> * bs_new must not be attached to a BlockBackend.
> *
> * This function does not create any image files.
> + *
> + * bdrv_append() takes ownership of a bs_new reference and unrefs it because
> + * that's what the callers commonly need. bs_new will be referenced by the
> old
> + * parents of bs_top after bdrv_append() returns. If the caller needs to
> keep a
> + * reference of its own, it must call bdrv_ref().
> */
> void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top)
> {
> - bdrv_swap(bs_new, bs_top);
> + assert(!bdrv_requests_pending(bs_top));
> + assert(!bdrv_requests_pending(bs_new));
> +
> + bdrv_ref(bs_top);
> + change_parent_backing_link(bs_top, bs_new);
> +
> + /* Some fields always stay on top of the backing file chain */
> + swap_feature_fields(bs_top, bs_new);
> +
> + bdrv_set_backing_hd(bs_new, bs_top);
> + bdrv_unref(bs_top);
>
> - /* The contents of 'tmp' will become bs_top, as we are
> - * swapping bs_new and bs_top contents. */
> - bdrv_set_backing_hd(bs_top, bs_new);
> + /* bs_new is now referenced by its new parents, we don't need the
> + * additional reference any more. */
> + bdrv_unref(bs_new);
> }
Before, all pointers to @bs_new were moved to @bs_top. Now, they stay at
@bs_new. I suppose we are assuming there are no pointers to @bs_new,
should we assert that, and/or point it out in the documentation?
Max
>
> static void bdrv_delete(BlockDriverState *bs)
>
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- Re: [Qemu-devel] [PATCH 11/16] block-backend: Add blk_set_bs(), (continued)
- Re: [Qemu-devel] [PATCH 13/16] block: Implement bdrv_append() without bdrv_swap(),
Max Reitz <=
[Qemu-devel] [PATCH 12/16] block: Introduce parents list, Kevin Wolf, 2015/09/17
[Qemu-devel] [PATCH 15/16] block: Add and use bdrv_replace_in_backing_chain(), Kevin Wolf, 2015/09/17
[Qemu-devel] [PATCH 05/16] block: Convert bs->file to BdrvChild, Kevin Wolf, 2015/09/17