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Re: [Qemu-devel] [PATCH v2] block: support dropping active in bdrv_drop_
From: |
Jeff Cody |
Subject: |
Re: [Qemu-devel] [PATCH v2] block: support dropping active in bdrv_drop_intermediate |
Date: |
Thu, 24 Oct 2013 06:19:13 -0400 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Tue, Oct 15, 2013 at 03:25:00PM +0800, Fam Zheng wrote:
> There is only one failure point: bdrv_change_backing_file in this
> function, so we can drop the qlist and try to change the backing file
> before deleting anything.
>
> This way bdrv_drop_intermediate is simplified while keeping the
> operation transactional. A bonus is dropping an active BDS is supported
> too by swapping the base and top. Although no caller uses this yet, the
> comment is updated to reflect the change.
>
> Signed-off-by: Fam Zheng <address@hidden>
>
> ---
> v2: check for active, top and base being in a backing chain. (Jeff)
This does check for that, but it doesn't catch all errors.
It will verify:
[base] -> [active]
And verifies:
[top] -> [active] (when active is != top)
However, it does not verify that the following is true:
[base] -> [top]
(e.g., it will pass on [top] -> [base] -> [active])
Rather than add another call to bdrv_find_overlay to verify the last
case, would just adding the bdrv_swap() and a check for active ==
top to the existing function do what you need for the active layer
support?
> Signed-off-by: Fam Zheng <address@hidden>
> ---
> block.c | 103
> ++++++++++++++++++++-------------------------------------
> block/commit.c | 1 +
> 2 files changed, 37 insertions(+), 67 deletions(-)
>
> diff --git a/block.c b/block.c
> index fd05a80..9ead554 100644
> --- a/block.c
> +++ b/block.c
> @@ -2130,18 +2130,11 @@ BlockDriverState *bdrv_find_overlay(BlockDriverState
> *active,
> return overlay;
> }
>
> -typedef struct BlkIntermediateStates {
> - BlockDriverState *bs;
> - QSIMPLEQ_ENTRY(BlkIntermediateStates) entry;
> -} BlkIntermediateStates;
> -
> -
> /*
> - * Drops images above 'base' up to and including 'top', and sets the image
> - * above 'top' to have base as its backing file.
> - *
> - * Requires that the overlay to 'top' is opened r/w, so that the backing file
> - * information in 'bs' can be properly updated.
> + * Drops images above 'base' up to and including 'top', and sets new 'base'
> + * as backing_hd of top_overlay (the image orignally has 'top' as backing
> + * file). top_overlay may be NULL if 'top' is active, no such update needed.
> + * Requires that the top_overlay to 'top' is opened r/w.
> *
> * E.g., this will convert the following chain:
> * bottom <- base <- intermediate <- top <- active
> @@ -2158,86 +2151,62 @@ typedef struct BlkIntermediateStates {
> *
> * base <- active
> *
> - * Error conditions:
> - * if active == top, that is considered an error
> + * It also allows active==top, in which case it converts:
> + *
> + * base <- intermediate <- active (also top)
> + *
> + * to
> + *
> + * base == active == top, i.e. only base remains: *top == *base when return.
> *
> */
> int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
> BlockDriverState *base)
> {
> - BlockDriverState *intermediate;
> + BlockDriverState *pbs;
> + BlockDriverState *overlay = NULL;
> BlockDriverState *base_bs = NULL;
> - BlockDriverState *new_top_bs = NULL;
> - BlkIntermediateStates *intermediate_state, *next;
> - int ret = -EIO;
> -
> - QSIMPLEQ_HEAD(states_to_delete, BlkIntermediateStates) states_to_delete;
> - QSIMPLEQ_INIT(&states_to_delete);
> + int ret = -EINVAL;
>
> if (!top->drv || !base->drv) {
> goto exit;
> }
>
> - new_top_bs = bdrv_find_overlay(active, top);
> -
> - if (new_top_bs == NULL) {
> - /* we could not find the image above 'top', this is an error */
> - goto exit;
> - }
> -
> - /* special case of new_top_bs->backing_hd already pointing to base -
> nothing
> - * to do, no intermediate images */
> - if (new_top_bs->backing_hd == base) {
> - ret = 0;
> + if (!bdrv_find_overlay(active, base)) {
> goto exit;
> }
>
> - intermediate = top;
> -
> - /* now we will go down through the list, and add each BDS we find
> - * into our deletion queue, until we hit the 'base'
> - */
> - while (intermediate) {
> - intermediate_state = g_malloc0(sizeof(BlkIntermediateStates));
> - intermediate_state->bs = intermediate;
> - QSIMPLEQ_INSERT_TAIL(&states_to_delete, intermediate_state, entry);
> -
> - if (intermediate->backing_hd == base) {
> - base_bs = intermediate->backing_hd;
> - break;
> + if (active != top) {
> + /* If there's an overlay, its backing_hd points to top's BDS now,
> + * the top image is dropped but this BDS structure is kept and
> swapped
> + * with base, this way we keep the pointers valid after dropping top
> */
> + overlay = bdrv_find_overlay(active, top);
> + if (!overlay) {
> + goto exit;
> + }
> + ret = bdrv_change_backing_file(overlay, base->filename,
> + base->drv ?
> + base->drv->format_name : "");
> + if (ret) {
> + goto exit;
> }
> - intermediate = intermediate->backing_hd;
> - }
> - if (base_bs == NULL) {
> - /* something went wrong, we did not end at the base. safely
> - * unravel everything, and exit with error */
> - goto exit;
> }
>
> - /* success - we can delete the intermediate states, and link top->base */
> - ret = bdrv_change_backing_file(new_top_bs, base_bs->filename,
> - base_bs->drv ? base_bs->drv->format_name
> : "");
> - if (ret) {
> - goto exit;
> + for (pbs = top->backing_hd; pbs != base; pbs = base_bs) {
> + assert(pbs);
> + base_bs = pbs->backing_hd;
> + pbs->backing_hd = NULL;
> + bdrv_unref(pbs);
> }
> - new_top_bs->backing_hd = base_bs;
> -
>
> - QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry,
> next) {
> - /* so that bdrv_close() does not recursively close the chain */
> - intermediate_state->bs->backing_hd = NULL;
> - bdrv_unref(intermediate_state->bs);
> - }
> - ret = 0;
> + bdrv_swap(base, top);
>
> + base->backing_hd = NULL;
> + bdrv_unref(base);
> exit:
> - QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry,
> next) {
> - g_free(intermediate_state);
> - }
> return ret;
> }
>
> -
> static int bdrv_check_byte_request(BlockDriverState *bs, int64_t offset,
> size_t size)
> {
> diff --git a/block/commit.c b/block/commit.c
> index d4090cb..4d8cd05 100644
> --- a/block/commit.c
> +++ b/block/commit.c
> @@ -142,6 +142,7 @@ wait:
> if (!block_job_is_cancelled(&s->common) && sector_num == end) {
> /* success */
> ret = bdrv_drop_intermediate(active, top, base);
> + base = top;
> }
>
> exit_free_buf:
> --
> 1.8.3.1
>
>