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Re: [Qemu-devel] [PATCH v15 08/14] block: Support dropping active in bdr
From: |
Jeff Cody |
Subject: |
Re: [Qemu-devel] [PATCH v15 08/14] block: Support dropping active in bdrv_drop_intermediate |
Date: |
Wed, 26 Feb 2014 14:42:12 -0500 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Sun, Feb 23, 2014 at 09:54:49AM +0800, Fam Zheng wrote:
> Dropping intermediate could be useful both for commit and stream, and
> BDS refcnt plus bdrv_swap could do most of the job nicely. It also needs
> to work with op blockers.
>
> Signed-off-by: Fam Zheng <address@hidden>
> ---
> block.c | 137
> +++++++++++++++++++++++----------------------------------
> block/commit.c | 2 +-
> 2 files changed, 56 insertions(+), 83 deletions(-)
>
> diff --git a/block.c b/block.c
> index a8d1012..13cbdbf 100644
> --- a/block.c
> +++ b/block.c
> @@ -2495,115 +2495,88 @@ 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.
> + * Drops images above 'base' up to and including 'top', and sets new 'base'
> as
> + * backing_hd of top's overlay (the image orignally has 'top' as backing
> file).
> + * top's overlay may be NULL if 'top' is active, no such update needed.
> + * Requires that the top's overlay to 'top' is opened r/w.
> + *
> + * 1) This will convert the following chain:
> + *
> + * ... <- base <- ... <- top <- overlay <-... <- active
> + *
> + * to
> + *
> + * ... <- base <- overlay <- active
> *
> - * Requires that the overlay to 'top' is opened r/w, so that the backing file
> - * information in 'bs' can be properly updated.
> + * 2) It is allowed for bottom==base, in which case it converts:
> *
> - * E.g., this will convert the following chain:
> - * bottom <- base <- intermediate <- top <- active
> + * base <- ... <- top <- overlay <- ... <- active
> *
> * to
> *
> - * bottom <- base <- active
> + * base <- overlay <- active
> *
> - * It is allowed for bottom==base, in which case it converts:
> + * 2) It also allows active==top, in which case it converts:
> *
> - * base <- intermediate <- top <- active
> + * ... <- base <- ... <- top (active)
> *
> * to
> *
> - * base <- active
> + * ... <- base == active == top
> + *
> + * i.e. only base and lower remains: *top == *base when return.
> + *
> + * 3) If base==NULL, it will drop all the BDS below overlay and set its
> + * backing_hd to NULL. I.e.:
> + *
> + * base(NULL) <- ... <- overlay <- ... <- active
> + *
> + * to
> *
> - * Error conditions:
> - * if active == top, that is considered an error
> + * overlay <- ... <- active
> *
> */
> int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
> BlockDriverState *base)
> {
> - BlockDriverState *intermediate;
> - 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);
> -
> - if (!top->drv || !base->drv) {
> - goto exit;
> - }
> + BlockDriverState *drop_start, *overlay;
> + int ret = -EINVAL;
>
> - 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 */
> + if (!top->drv || (base && !base->drv)) {
> 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) {
> + if (top == base) {
> ret = 0;
> - 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;
> + } else if (top == active) {
> + assert(base);
Hmm. I'm trying to think of any edge cases here, if something could
fail. We know base != NULL, we know that top = active, and that
top != base. That is good. But we don't have any verification at
this point that base is in the same chain as top / active.
Is this worth verifying? I think it probably is... a simple way
would be to just add here something like this:
drop_start = active;
while (drop_start && drop_start != base) {
drop_start = drop_start->backing_hd;
}
if (drop_start == NULL) {
goto exit;
}
(or use a new BDS pointer for better clarity, rather than reusing
drop_start, if desired)
> + drop_start = active->backing_hd;
> + bdrv_swap(active, base);
> + base->backing_hd = NULL;
> + bdrv_unref(drop_start);
> + ret = 0;
> + } else if (base) {
> + overlay = bdrv_find_overlay(active, top);
Error check needed - overlay could be NULL.
> + bdrv_set_backing_hd(overlay, base);
> + ret = 0;
This breaks block-commit here, because the backing file is not changed
in the overlay image file.
> + } else {
> + /* 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;
> }
> - 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;
> - }
> - new_top_bs->backing_hd = base_bs;
> -
> - bdrv_refresh_limits(new_top_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 = bdrv_change_backing_file(overlay, NULL, NULL);
> + if (ret) {
> + goto exit;
> + }
As mentioned above, the backing file needs to be changed in each case
(except the top == base case), not just when base == NULL.
> + bdrv_set_backing_hd(overlay, NULL);
> }
> - ret = 0;
> -
> 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 acec4ac..ac598d6 100644
> --- a/block/commit.c
> +++ b/block/commit.c
> @@ -73,6 +73,7 @@ static void coroutine_fn commit_run(void *opaque)
> int bytes_written = 0;
> int64_t base_len;
>
> + overlay_bs = bdrv_find_overlay(active, top);
> ret = s->common.len = bdrv_getlength(top);
>
>
> @@ -154,7 +155,6 @@ exit_restore_reopen:
> if (s->base_flags != bdrv_get_flags(base)) {
> bdrv_reopen(base, s->base_flags, NULL);
> }
> - overlay_bs = bdrv_find_overlay(active, top);
> if (overlay_bs && s->orig_overlay_flags != bdrv_get_flags(overlay_bs)) {
> bdrv_reopen(overlay_bs, s->orig_overlay_flags, NULL);
> }
> --
> 1.8.5.4
>
>
- [Qemu-devel] [PATCH v15 00/14] Drop in_use from BlockDriverState and enable point-in-time snapshot exporting over NBD, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 01/14] block: Add BlockOpType enum, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 02/14] block: Introduce op_blockers to BlockDriverState, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 03/14] block: Replace in_use with operation blocker, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 04/14] block: Move op_blocker check from block_job_create to its caller, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 05/14] block: Add bdrv_set_backing_hd(), Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 06/14] block: Add backing_blocker in BlockDriverState, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 07/14] block: Parse "backing" option to reference existing BDS, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 08/14] block: Support dropping active in bdrv_drop_intermediate, Fam Zheng, 2014/02/22
- Re: [Qemu-devel] [PATCH v15 08/14] block: Support dropping active in bdrv_drop_intermediate,
Jeff Cody <=
- [Qemu-devel] [PATCH v15 09/14] stream: Use bdrv_drop_intermediate and drop close_unused_images, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 10/14] qmp: Add command 'blockdev-backup', Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 11/14] block: Allow backup on referenced named BlockDriverState, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 12/14] block: Add blockdev-backup to transaction, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 13/14] qemu-iotests: Test blockdev-backup in 055, Fam Zheng, 2014/02/22
- [Qemu-devel] [PATCH v15 14/14] qemu-iotests: Image fleecing test case 083, Fam Zheng, 2014/02/22