Re: [Qemu-devel] [PATCH v2 2/7] block: add live block commit functionality

[Jeff Cody <jcody@redhat.com>]

On 09/25/2012 02:12 PM, Eric Blake wrote:
> On 09/25/2012 10:29 AM, Jeff Cody wrote:
>> This adds the live commit coroutine.  This iteration focuses on the
>> commit only below the active layer, and not the active layer itself.
>>
>> The behaviour is similar to block streaming; the sectors are walked
>> through, and anything that exists above 'base' is committed back down
>> into base.  At the end, intermediate images are deleted, and the
>> chain stitched together.  Images are restored to their original open
>> flags upon completion.
>>
>> Signed-off-by: Jeff Cody <jcody@redhat.com>
> 
>> +void commit_start(BlockDriverState *bs, BlockDriverState *base,
>> +                 BlockDriverState *top, int64_t speed,
>> +                 BlockErrorAction on_error, BlockDriverCompletionFunc *cb,
>> +                 void *opaque, Error **errp)
>> +{
>> +    CommitBlockJob *s;
>> +    BlockReopenQueue *reopen_queue = NULL;
>> +    int orig_overlay_flags;
>> +    int orig_base_flags;
>> +    BlockDriverState *overlay_bs;
>> +    Error *local_err = NULL;
>> +
>> +    if ((on_error == BLOCK_ERR_STOP_ANY ||
>> +         on_error == BLOCK_ERR_STOP_ENOSPC) &&
>> +        !bdrv_iostatus_is_enabled(bs)) {
>> +        error_set(errp, QERR_INVALID_PARAMETER_COMBINATION);
>> +        return;
>> +    }
>> +
>> +    overlay_bs = bdrv_find_overlay(bs, top);
>> +
>> +    if (overlay_bs == NULL) {
>> +        error_setg(errp, "Could not find overlay image for %s:", top->filename);
>> +        return;
>> +    }
>> +
>> +    orig_base_flags    = bdrv_get_flags(base);
>> +    orig_overlay_flags = bdrv_get_flags(overlay_bs);
> 
> I think you are missing a check here that base is on the backing chain
> of top.  See also my comments to 5/7.
> 

Did you mean your comments on 6/7 (or am I missing an email)?

This does get partially validated in patch 5/7, in the
qmp_block_commit() handler - both base and top are verified to be in the
chain 'bs'.  What is not validated, however, is that you did not swap
your 'top' and 'base' arguments.  I'll add a check here for that, to
make sure that base is reachable from overlay_bs.


>> +
>> +    /* convert base_bs & overlay_bs to r/w, if necessary */
>> +    if (!(orig_base_flags & BDRV_O_RDWR)) {
>> +        reopen_queue = bdrv_reopen_queue(reopen_queue, base,
>> +                                         orig_base_flags | BDRV_O_RDWR);
>> +    }
>> +    if (!(orig_overlay_flags & BDRV_O_RDWR)) {
>> +        reopen_queue = bdrv_reopen_queue(reopen_queue, overlay_bs,
>> +                                         orig_overlay_flags | BDRV_O_RDWR);
>> +    }
>> +    if (reopen_queue) {
>> +        bdrv_reopen_multiple(reopen_queue, &local_err);
> 
> Is it valid to make a no-op call, such as:
> 
> { "execute":"block-commit", "arguments":{
>   "device":"drive0", "top":"base", "base":"base" }}
> 
> If so, should we do an early exit here, rather than temporarily changing
> base to R/W just to change it back to R/O?
> 
> If not, should we be rejecting it up front (again, back to the question
> of failing if 'base' is not a backing file of 'top', even if both 'top'
> and 'base' are backing files of 'device').
> 

I'll add a quick check for that as well.  Patch 5 checks for it in one
scenario (default 'top'), and returns a generic error of:
"Invalid files for merge: top and base are the same"

I'll make sure to check for that in all scenarios (or just make the test
mentioned earlier incorporate top == base as well)

Thanks,
Jeff