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

[Changheun Lee]

> 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?

Actually I didn't reproduced data corruption. I only reproduced hang during
making ext4 filesystem. Alex, could you check it?

> 
> > 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