Re: [Qemu-devel] [PATCH v5 14/42] block: Use CAFs when working with backing chains

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

13.06.2019 1:09, Max Reitz wrote:
> Use child access functions when iterating through backing chains so
> filters do not break the chain.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>   block.c | 40 ++++++++++++++++++++++++++++------------
>   1 file changed, 28 insertions(+), 12 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 11f37983d9..505b3e9a01 100644
> --- a/block.c
> +++ b/block.c
> @@ -4261,7 +4261,8 @@ int bdrv_change_backing_file(BlockDriverState *bs,
>   }
>   
>   /*
> - * Finds the image layer in the chain that has 'bs' as its backing file.
> + * Finds the image layer in the chain that has 'bs' (or a filter on
> + * top of it) as its backing file.
>    *
>    * active is the current topmost image.
>    *
> @@ -4273,11 +4274,18 @@ int bdrv_change_backing_file(BlockDriverState *bs,
>   BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
>                                       BlockDriverState *bs)
>   {
> -    while (active && bs != backing_bs(active)) {
> -        active = backing_bs(active);
> +    bs = bdrv_skip_rw_filters(bs);
> +    active = bdrv_skip_rw_filters(active);
> +
> +    while (active) {
> +        BlockDriverState *next = bdrv_backing_chain_next(active);
> +        if (bs == next) {
> +            return active;
> +        }
> +        active = next;
>       }
>   
> -    return active;
> +    return NULL;
>   }

Semantics changed for this function.
It is used in two places
1. from bdrv_find_base wtih @bs=NULL, it should be unchanged, as I hope we will never have
    filter node as a bottom of some valid chain

2. from qmp_block_commit, only to check op-blocker... hmmm. I really don't understand,
why do we check BLOCK_OP_TYPE_COMMIT_TARGET on top_bs overlay.. top_bs overlay is out of the job,
what is this check for?


>   
>   /* Given a BDS, searches for the base layer. */
> @@ -4421,9 +4429,7 @@ int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
>        * other intermediate nodes have been dropped.
>        * If 'top' is an implicit node (e.g. "commit_top") we should skip
>        * it because no one inherits from it. We use explicit_top for that. */
> -    while (explicit_top && explicit_top->implicit) {
> -        explicit_top = backing_bs(explicit_top);
> -    }
> +    explicit_top = bdrv_skip_implicit_filters(explicit_top);
>       update_inherits_from = bdrv_inherits_from_recursive(base, explicit_top);
>   
>       /* success - we can delete the intermediate states, and link top->base */
> @@ -4902,7 +4908,7 @@ BlockDriverState *bdrv_lookup_bs(const char *device,
>   bool bdrv_chain_contains(BlockDriverState *top, BlockDriverState *base)
>   {
>       while (top && top != base) {
> -        top = backing_bs(top);
> +        top = bdrv_filtered_bs(top);
>       }
>   
>       return top != NULL;
> @@ -5141,7 +5147,17 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
>   
>       is_protocol = path_has_protocol(backing_file);
>   
> -    for (curr_bs = bs; curr_bs->backing; curr_bs = curr_bs->backing->bs) {
> +    /*
> +     * Being largely a legacy function, skip any filters here
> +     * (because filters do not have normal filenames, so they cannot
> +     * match anyway; and allowing json:{} filenames is a bit out of
> +     * scope).
> +     */
> +    for (curr_bs = bdrv_skip_rw_filters(bs);
> +         bdrv_filtered_cow_child(curr_bs) != NULL;
> +         curr_bs = bdrv_backing_chain_next(curr_bs))
> +    {
> +        BlockDriverState *bs_below = bdrv_backing_chain_next(curr_bs);
>   
>           /* If either of the filename paths is actually a protocol, then
>            * compare unmodified paths; otherwise make paths relative */
> @@ -5149,7 +5165,7 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
>               char *backing_file_full_ret;
>   
>               if (strcmp(backing_file, curr_bs->backing_file) == 0) {

hmm, interesting, what bs->backing_file now means? It's strange enough to store such field on
bds, when we have backing link anyway..

> -                retval = curr_bs->backing->bs;
> +                retval = bs_below;
>                   break;
>               }
>               /* Also check against the full backing filename for the image */
> @@ -5159,7 +5175,7 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
>                   bool equal = strcmp(backing_file, backing_file_full_ret) == 0;
>                   g_free(backing_file_full_ret);
>                   if (equal) {
> -                    retval = curr_bs->backing->bs;
> +                    retval = bs_below;
>                       break;
>                   }
>               }
> @@ -5185,7 +5201,7 @@ BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
>               g_free(filename_tmp);
>   
>               if (strcmp(backing_file_full, filename_full) == 0) {
> -                retval = curr_bs->backing->bs;
> +                retval = bs_below;
>                   break;
>               }
>           }
> 


-- 
Best regards,
Vladimir