Re: [Qemu-devel] Safely reopening image files by stashing fds

[Kevin Wolf <kwolf@redhat.com>]

Am 09.08.2011 12:25, schrieb Stefan Hajnoczi:
> On Mon, Aug 8, 2011 at 4:16 PM, Kevin Wolf <kwolf@redhat.com> wrote:
>> Am 08.08.2011 16:49, schrieb Stefan Hajnoczi:
>>> On Fri, Aug 5, 2011 at 10:48 AM, Kevin Wolf <kwolf@redhat.com> wrote:
>>>> Am 05.08.2011 11:29, schrieb Stefan Hajnoczi:
>>>>> On Fri, Aug 5, 2011 at 10:07 AM, Kevin Wolf <kwolf@redhat.com> wrote:
>>>>>> Am 05.08.2011 10:40, schrieb Stefan Hajnoczi:
>>>>>>> We've discussed safe methods for reopening image files (e.g. useful for
>>>>>>> changing the hostcache parameter).  The problem is that closing the file first
>>>>>>> and then opening it again exposes us to the error case where the open fails.
>>>>>>> At that point we cannot get to the file anymore and our options are to
>>>>>>> terminate QEMU, pause the VM, or offline the block device.
>>>>>>>
>>>>>>> This window of vulnerability can be eliminated by keeping the file descriptor
>>>>>>> around and falling back to it should the open fail.
>>>>>>>
>>>>>>> The challenge for the file descriptor approach is that image formats, like
>>>>>>> VMDK, can span multiple files.  Therefore the solution is not as simple as
>>>>>>> stashing a single file descriptor and reopening from it.
>>>>>>
>>>>>> So far I agree. The rest I believe is wrong because you can't assume
>>>>>> that every backend uses file descriptors. The qemu block layer is based
>>>>>> on BlockDriverStates, not fds. They are a concept that should be hidden
>>>>>> in raw-posix.
>>>>>>
>>>>>> I think something like this could do:
>>>>>>
>>>>>> struct BDRVReopenState {
>>>>>>    BlockDriverState *bs;
>>>>>>    /* can be extended by block drivers */
>>>>>> };
>>>>>>
>>>>>> .bdrv_reopen(BlockDriverState *bs, BDRVReopenState **reopen_state, int
>>>>>> flags);
>>>>>> .bdrv_reopen_commit(BDRVReopenState *reopen_state);
>>>>>> .bdrv_reopen_abort(BDRVReopenState *reopen_state);
>>>>>>
>>>>>> raw-posix would store the old file descriptor in its reopen_state. On
>>>>>> commit, it closes the old descriptors, on abort it reverts to the old
>>>>>> one and closes the newly opened one.
>>>>>>
>>>>>> Makes things a bit more complicated than the simple bdrv_reopen I had in
>>>>>> mind before, but it allows VMDK to get an all-or-nothing semantics.
>>>>>
>>>>> Can you show how bdrv_reopen() would use these new interfaces?  I'm
>>>>> not 100% clear on the idea.
>>>>
>>>> Well, you wouldn't only call bdrv_reopen, but also either
>>>> bdrv_reopen_commit/abort (for the top-level caller we can have a wrapper
>>>> function that does both, but that's syntactic sugar).
>>>>
>>>> For example we would have:
>>>>
>>>> int vmdk_reopen()
>>>
>>> .bdrv_reopen() is a confusing name for this operation because it does
>>> not reopen anything.  bdrv_prepare_reopen() might be clearer.
>>
>> Makes sense.
>>
>>>
>>>> {
>>>>    *((VMDKReopenState**) rs) = malloc();
>>>>
>>>>    foreach (extent in s->extents) {
>>>>        ret = bdrv_reopen(extent->file, &extent->reopen_state)
>>>>        if (ret < 0)
>>>>            goto fail;
>>>>    }
>>>>    return 0;
>>>>
>>>> fail:
>>>>    foreach (extent in rs->already_reopened) {
>>>>        bdrv_reopen_abort(extent->reopen_state);
>>>>    }
>>>>    return ret;
>>>> }
>>>
>>>> void vmdk_reopen_commit()
>>>> {
>>>>    foreach (extent in s->extents) {
>>>>        bdrv_reopen_commit(extent->reopen_state);
>>>>    }
>>>>    free(rs);
>>>> }
>>>>
>>>> void vmdk_reopen_abort()
>>>> {
>>>>    foreach (extent in s->extents) {
>>>>        bdrv_reopen_abort(extent->reopen_state);
>>>>    }
>>>>    free(rs);
>>>> }
>>>
>>> Does the caller invoke bdrv_close(bs) after bdrv_prepare_reopen(bs,
>>> &rs)?
>>
>> No. Closing the old backend would be part of bdrv_reopen_commit.
>>
>> Do you have a use case where it would be helpful if the caller invoked
>> bdrv_close?
> 
> When the caller does bdrv_close() two BlockDriverStates are never open
> for the same image file.  I thought this was a property we wanted.
> 
> Also, in the block_set_hostcache case we need to reopen without
> switching to a new BlockDriverState instance.  That means the reopen
> needs to be in-place with respect to the BlockDriverState *bs pointer.
>  We cannot create a new instance.

Yes, but where do you even get the second BlockDriverState from?

My prototype only returns an int, not a new BlockDriverState. Until
bdrv_reopen_commit() it would refer to the old file descriptors etc. and
after bdrv_reopen_commit() the very same BlockDriverState would refer to
the new ones.

Kevin