linux-ext4 - Re: minix/ext2 + rd problem

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Date:	Wed, 15 Oct 2008 16:10:08 +0200 (CEST)
From:	Richard Kojedzinszky <krichy@...etwork.hu>
To:	Nick Piggin <npiggin@...e.de>
cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: minix/ext2 + rd problem

Dear Nick,

Sorry for my stupid question, but how can i flush a blockdev? If i can 
do it without unmounting the fs i will be happy.

Thanks in advance,


Kojedzinszky Richard
TvNetWork Nyrt.
E-mail: krichy (at) tvnetwork [dot] hu
PGP: 0x24E79141
   Fingerprint = 6847 ECFF EF58 0C09 18A5  16CF 270F 0C6F 24E7 9141

On Wed, 15 Oct 2008, Nick Piggin wrote:

> Date: Wed, 15 Oct 2008 16:05:23 +0200
> From: Nick Piggin <npiggin@...e.de>
> To: Richard Kojedzinszky <krichy@...etwork.hu>
> Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
> Subject: Re: minix/ext2 + rd problem
> 
> On Wed, Oct 15, 2008 at 10:19:44AM +0200, Richard Kojedzinszky wrote:
>> dear nick,
>>
>> i have tried a sync after the remount, but that did not help. what helped
>> is dropping the cache by echoing 3 to /proc/sys/vm/drop_caches, but this
>> still didnt solve the problem in 100%, only in 95% of the cases.
>>
>> But when i read the device with
>> # dd if=/dev/ram0 iflag=direct ...
>> then it worked. I think this bypassed some caches, and thus read the
>> actual data.
>>
>> But a sad result is that I experienced with it, and only with ramdisk does
>> it work as expected. for example with a logical volume it behaves in the
>> wrong way.
>
> I've reproduced this problem (ext2 image corruption flagged in e2fsck
> even though it was remounted ro and marked clean in the sb).
>
> Issuing a sync, then drop_caches, seems to fix it here for me.
>
> On the other hand, I also see problems with inconsistencies even after
> unmounting if I hold the /dev/ram0 device open with something else (which
> causes the buffer cache not to be invalidated on unmount).
>
> I think what is happening is that the block device is being modified
> without going through the buffer cache (ie. via pagecache or direct
> writes), but the buffer cache doesn't get invalidated. So you get stale
> data when reading from /dev/ram0.
>
> I don't think we're going to want the overhead in the kernel to detect
> these kinds of aliases. It might be reasonable to flush the blockdev
> on unmount and remount,ro after syncing the filesystem.
>
> The old rd driver's backing store was actually its buffercache, so that
> particular issue wouldn't cause aliasing.
>
> Thanks,
> Nick
>
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