Re: Is it possible to corrupt disk when writeback page with undetected UE?

[Yang Shi <shy828301@gmail.com>]

On Thu, Sep 15, 2022 at 5:27 PM Jane Chu <jane.chu@oracle.com> wrote:
>
> On 9/15/2022 3:50 PM, Luck, Tony wrote:
> >> Suppose there is a UE in a DRAM page that is backed by a disk file.
> >> The UE hasn't been reported to the kernel, but low level firmware
> >> initiated scrubbing has already logged the UE.
> >>
> >> The page is then dirtied by a write, although the write clearly failed,
> >> it didn't trigger an MCE.
> >>
> >> And without a subsequent read from the page, at some point, the page is
> >> written back to the disk, leaving a PAGE_SIZE of zeros in the targeted
> >> disk blocks.
> >>
> >> Is this mode of disk corruption possible?
> >
> > I didn't look at what was written to disk, but I have seen this. My test sequence
> > was to compile and then immediately run an error injection test program that
> > injected a memory UC error to an instruction.
> >
> > Because the program was freshly compiled, the executable file was in the
> > page cache with all pages marked as modified. Later a sync (or memory
> > pressure) wrote the dirty page with poison to filesystem.
> >
> > I did see an error reported by the disk controller.
>
> Thanks a lot for this information!
>
> Were you using madvise to inject an error to a mmap'ed address?
> or a different tool?  Do you still have the test documented
> somewhere?
>
> And, aside from verifying every write with a read prior to sync,
> any suggestion to minimize the window of such corruption?

We discussed the topic at this year's LSFMM summit. Please refer to
https://lwn.net/Articles/893565/

>
> thanks!
> -jane
>
> >
> > -Tony
>