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

[Jane Chu <jane.chu@oracle.com>]

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?

thanks!
-jane

> 
> -Tony