linux-ext4 - Re: regression: data corruption with ext4 on LUKS on nvme with torvalds master

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Date:   Fri, 14 May 2021 05:50:40 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Milan Broz <gmazyland@...il.com>,
        Bart Van Assche <bvanassche@....org>,
        "Theodore Ts'o" <tytso@....edu>,
        Changheun Lee <nanich.lee@...sung.com>
cc:     alex_y_xu@...oo.ca, axboe@...nel.dk, bgoncalv@...hat.com,
        dm-crypt@...ut.de, hch@....de, jaegeuk@...nel.org,
        linux-block@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        ming.lei@...hat.com, yi.zhang@...hat.com, dm-devel@...hat.com
Subject: Re: regression: data corruption with ext4 on LUKS on nvme with
 torvalds master



On 5/13/21 7:15 AM, Theodore Ts'o wrote:
> On Thu, May 13, 2021 at 06:42:22PM +0900, Changheun Lee wrote:
>>
>> Problem might be casued by exhausting of memory. And memory exhausting
>> would be caused by setting of small bio_max_size. Actually it was not
>> reproduced in my VM environment at first. But, I reproduced same problem
>> when bio_max_size is set with 8KB forced. Too many bio allocation would
>> be occurred by setting of 8KB bio_max_size.
> 
> Hmm... I'm not sure how to align your diagnosis with the symptoms in
> the bug report.  If we were limited by memory, that should slow down
> the I/O, but we should still be making forward progress, no?  And a
> forced reboot should not result in data corruption, unless maybe there

If you use data=writeback, data writes and journal writes are not 
synchronized. So, it may be possible that a journal write made it through, 
a data write didn't - the end result would be a file containing random 
contents that was on the disk.

Changheun - do you use data=writeback? Did the corruption happen only in 
newly created files? Or did it corrupt existing files?

> was a missing check for a failed memory allocation, causing data to be
> written to the wrong location, a missing error check leading to the
> block or file system layer not noticing that a write had failed
> (although again, memory exhaustion should not lead to failed writes;
> it might slow us down, sure, but if writes are being failed, something
> is Badly Going Wrong --- things like writes to the swap device or
> writes by the page cleaner must succeed, or else Things Would Go Bad
> In A Hurry).

Mikulas