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

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Date:	Tue, 12 Jan 2010 14:12:39 +0100
From:	Michal Novotny <minovotn@...hat.com>
To:	Ric Wheeler <rwheeler@...hat.com>
CC:	Christoph Hellwig <hch@...radead.org>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] extend e2fsprogs functionality to add EXT2_FLAG_DIRECT
 option

On 01/12/2010 02:04 PM, Ric Wheeler wrote:
> On 01/12/2010 08:01 AM, Michal Novotny wrote:
>> On 01/12/2010 01:46 PM, Christoph Hellwig wrote:
>>> On Tue, Jan 12, 2010 at 01:30:40PM +0100, Michal Novotny wrote:
>>>> Not really, pygrub doesn't do any manipulation with file system and
>>>> also, it's not working on a life file system. It's called before the
>>>> guest boots up to read information about grub.conf/initrd and 
>>>> kernel for
>>>> PV guest and after this is read and selected in pygrub then the 
>>>> guest is
>>>> booted using the kernel and initrd extracted from the image (after 
>>>> which
>>>> the file is closed). Once again, nothing uses write support and it was
>>>> added just to make it use O_DIRECT for both read and write operations
>>>> but only pygrub uses only read support and O_DIRECT passed here is the
>>>> only way to make it use non-cached data.
>>> So what caches get in the way? From the above it seems the situation
>>> is the following:
>>>
>>> - filesystem N is a guest filesystem. It's not usually mounted on the
>>> host, except for initial setup long time ago
>>
>> Yes, it is really a guest file system. This is not mounted in the host
>> and the reason is to get actual version of grub.conf, initrd and kernel
>> to be booted...
>>
>>> - before booting a guest your "pygrub" tools needs to read files on
>>> it, and it's doing so using e2fsprogs
>>
>> Correct.
>>
>>> - once the guest is life it uses the extN kernel driver to access the
>>> filesystem
>>
>> That's right. So this is no longer pygrub responsibility...
>>
>>> nowhere in this cycle you should have any stale cached data. The kernel
>>> always makes sure to write back data on umount/reboot, as does 
>>> e2fsprogs
>>> if actually used to write data (which you said is not the case anyway).
>>
>> In fact I was unable to run into those problems myself but
>> reporter/customer did.
>>
>>> The only data that may be in the cache are unmodified data from reads
>>> on the block device from either e2fsprogs or a suboptimal virtual block
>>> device implementation, but these can't cause any problems.
>> Michal
>
> If the guest is the only one (when running) that installs a new 
> grub.conf file and kernel and it shuts down properly, you should be 
> good. It if does not shut down cleanly, it could have a stale 
> grub.conf file (or worse, a partially written one), but using O_DIRECT 
> to bypass the file system cache should not help.
>
> If we cannot reproduce this failure, sounds like we need to go back 
> and get a better understanding of what the customer saw?
>
> ric
>
That's right. I am going write an e-mail regarding this information to 
the reproducer if this bug and tell him that I need more information 
about what's happening at the customer side.

Michal
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