linux-ext4 - Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG

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Date:	Tue, 12 Jan 2010 10:56:06 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Michal Novotny <minovotn@...hat.com>
CC:	Ric Wheeler <rwheeler@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG_DIRECT
 option

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

> Thanks,
> Michal
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