On Mon, Mar 10, 2014 at 03:26:04PM +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 <f...@redhat.com>
> ---
> block.c | 139
> ++++++++++++++++++++++++++++-----------------------------
> block/commit.c | 2 +-
> 2 files changed, 70 insertions(+), 71 deletions(-)
>
> diff --git a/block.c b/block.c
> index 05f7766..0af7c62 100644
> --- a/block.c
> +++ b/block.c
> @@ -2503,115 +2503,114 @@ 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
> *
> - * Requires that the overlay to 'top' is opened r/w, so that the backing file
> - * information in 'bs' can be properly updated.
> + * to
> + *
> + * ... <- base <- overlay <- active
> + *
> + * 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.:
> *
> - * Error conditions:
> - * if active == top, that is considered an error
> + * base(NULL) <- ... <- overlay <- ... <- active
> + *
> + * to
> + *
> + * 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);
> + BlockDriverState *drop_start, *overlay, *bs;
> + int ret = -EINVAL;
>
> - if (!top->drv || !base->drv) {
> + assert(active);
> + assert(top);
> + /* Verify that top is in backing chain of active */
> + bs = active;
> + while (bs && bs != top) {
> + bs = bs->backing_hd;
> + }
> + if (!bs) {
> goto exit;
> }
> + /* Verify that base is in backing chain of top */
> + if (base) {
> + while (bs && bs != base) {
> + bs = bs->backing_hd;
> + }
> + if (bs != base) {
> + 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 */
> + 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;
> + } else if (top == active) {
> + assert(base);
> + drop_start = active->backing_hd;
> + bdrv_swap(active, base);
This will assert in block.c, in bdrv_swap, on the test for
anonymity of active. (For testing, I changed the active layer commit
in mirror to use bdrv_drop_intermediate()).
Unfortunately, there are other problems as well (anonymity could be
fixed by bdrv_make_anon(active)).
Using line numbers from my version of block.c, lines 1957, 1959, and
1960 will each cause an assert (these lines are all in bdrv_swap()):
1956: /* bs_new must be anonymous and shouldn't have anything fancy enabled
*/
1957: assert(bs_new->device_name[0] == '\0');
1958: assert(QLIST_EMPTY(&bs_new->dirty_bitmaps));
1959: assert(bs_new->job == NULL);
1960: assert(bs_new->dev == NULL);
Markus - on line 1960 above, is it safe to remove that check (and the
other check further down in bdrv_swap())?
Thinking about it more, there may be other landmines in bdrv_swap()
for this case; prior to this, bdrv_swap() was always called with
bs_new being a newly-created BDS. With the active layer BDS it almost
certainly is not 'new', and could have other "prohibited" fields set,
in addition to the above (e.g. io limits, etc.)
> + base->backing_hd = NULL;
> + bdrv_unref(drop_start);
> 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;
> - }
> - 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 */
> + overlay = bdrv_find_overlay(active, top);
> + if (!overlay) {
> 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
> : "");
> + ret = bdrv_change_backing_file(overlay,
> + base ? base->filename : NULL,
> + base ? base->drv->format_name : NULL);
> 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);
> + bs = overlay->backing_hd;
> + bdrv_set_backing_hd(overlay, base);
> + if (base) {
> + bdrv_ref(base);
> + }
> + if (bs) {
> + bdrv_unref(bs);
> }
> 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);
> }
> --
> 1.9.0
>
>
