Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG_DIRECT option

[Ric Wheeler <rwheeler@redhat.com>]

On 01/12/2010 11:56 AM, Eric Sandeen wrote:
> Michal Novotny wrote:
>> On 01/12/2010 05:50 PM, Ric Wheeler wrote:
>>> On 01/12/2010 11:43 AM, Michal Novotny wrote:
>>>> On 01/12/2010 05:38 PM, Christoph Hellwig wrote:
>>>>> Ok, I looked at the issue. The problem is that the Xen backend drivers
>>>>> are (as expected) utterly braindead and submit bios directly from the
>>>>> virtualization backed without using proper abstractions and thus
>>>>> bypassing all the cache coherency features in the fileystems (the block
>>>>> device nodes are just another mini-filesystem in that respect). So
>>>>> when you first have buffered access in the host pages may stay in cache
>>>>> and get overwritten directly on disk by a Xen guest, and once the guest
>>>>> is down the host may still use the now stale cached data.
>>>>>
>>>>> I would recommend to migrate your cutomers to KVM which uses the proper
>>>>> abtractions and thus doesn't have this problem. There's a reason after
>>>>> all why all the Xen dom0 mess never got merged to mainline.
>>>> So, do you think the problem is in the Xen backend drivers and to make
>>>> it working right in Xen the driver fix is needed?
>>>
>>> If XEN drivers by pass the normal IO and FS stack on the host, then I
>>> can understand why the hack to e2fsprogs works but it does not seem
>>> like a good fix.
>>>
>>> Specifically, the data will continue to be cached (and if dirty, might
>>> be written back to the storage eventually).
>>>
>>> If we need a work around, you need to drop VM caches for that device
>>> before you update the guest's files and possibly again afterwards (and
>>> make sure that nothing pulls the data into cache during the operation).
>>>
>>> Basically, this sounds like the backend drivers are doing something
>>> really, really dangerous....
>>>
>>> ric
>>>
>> Ok, so you think this is not good to do this patch for e2fsprogs for
>> direct access support? The only things we could do now is to fix the
>> backend drivers or create a workaround to drop caches? I need to discuss
>> this further with guys in my team...
>
> I do think that patching it up in e2fsprogs is unnecessarily invasive;
> it's fixing it at the wrong spot.
>
> Any block dev IO from the host is dangerous; fixing it only in e2fsprogs
> for this one case doesn't seem like the right course of action.
>
> -Eric

It actually could produce some nastier issues where it would work a bit, the bad 
data gets flushed back to the backing store and then your O_DIRECT read would be 
broken.

Also, for normal users of e2fsprogs, they should never bypass the cache...

ric