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

[Stefan Hajnoczi]

On Mon, Aug 8, 2011 at 4:16 PM, Kevin Wolf <address@hidden> wrote:
> Am 08.08.2011 16:49, schrieb Stefan Hajnoczi:
>> On Fri, Aug 5, 2011 at 10:48 AM, Kevin Wolf <address@hidden> wrote:
>>> Am 05.08.2011 11:29, schrieb Stefan Hajnoczi:
>>>> On Fri, Aug 5, 2011 at 10:07 AM, Kevin Wolf <address@hidden> 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.

Stefan