linux-ext4 - Re: Remounting filesystem read-only

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Message-ID: <91cab9ba274131c4701fe4c1ce444d6b@codeaurora.org>
Date:   Wed, 01 Aug 2018 19:23:10 -0700
From:   Sodagudi Prasad <psodagud@...eaurora.org>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>, adilger.kernel@...ger.ca,
        wen.xu@...ech.edu, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Remounting filesystem read-only

On 2018-07-28 00:47, Darrick J. Wong wrote:
> On Fri, Jul 27, 2018 at 08:18:23PM -0400, Theodore Y. Ts'o wrote:
>> On Fri, Jul 27, 2018 at 01:34:31PM -0700, Sodagudi Prasad wrote:
>> > > The error should be pretty clear: "Inode table for bg 0 marked as
>> > > needing zeroing".  That should never happen.
>> >
>> > Can you provide any debug patch to detect when this corruption is happening?
>> > Source of this corruption and how this is partition getting corrupted?
>> > Or which file system operation lead to this corruption?
>> 
>> Do you have a reliable repro?  If it's a one-off, it can be caused by
>> *anything*.  Crappy hardware, a bug in some proprietary, binary-only
>> GPU driver dereferencing some wild pointer that corrupts kernel
>> memory, etc.
>> 
>> Asking for a debug patch is like asking for "can you create technology
>> that can detect when a cockroach enter my house?"
> 
> Well, ext4 *could* add metadata read and write verifiers to complain
> loudly in dmesg about stuff that shouldn't be there, so at least we'd
> know when we're writing cockroaches into the house... :)
> 
> --D
> 
Hi Ted,

Below change fixed this issue. Thanks for your support.

https://github.com/torvalds/linux/commit/5012284700775a4e6e3fbe7eac4c543c4874b559

"ext4: fix check to prevent initializing reserved inodes"

-Thanks, Prasad

>> So if you have a reliable repro, then we know what operations might be
>> triggering the corruption, and then you work on creating a minimal
>> repro, and only *then* when we have a restricted set of possibilities
>> that might be the cause (for example, if removing a GPU call makes the
>> problem go away, then the patch would need to be in the proprietary
>> GPU driver....)
>> 
>> > I am digging code a bit around this warning to understand more.
>> 
>> The warning means that a flag in block group descriptor #0 is set
>> that should never be set.  How did the flag get set?  There is any
>> number of things that could cause that.
>> 
>> You might want to look at the block group descriptor via dumpe2fs or
>> debugfs, to see if it's just a single bit getting flipped, or if the
>> entire block group descriptor is garbage.  Note that under normal code
>> paths, the flag *never* gets set by ext4 kernel code.  The flag will
>> get set on non-block group 0 block group descriptors by ext4, and the
>> ext4 kernel code will only clear the flag.
>> 
>> Of course, if there is a bug in some driver that dereferences a
>> pointer widely, all bets are off.
>> 
>> 					- Ted

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