Error detection for microSD-based swap, buildworld failures on pi3

[new subject, different emphasis, old problem]

On Mon, Jan 31, 2022 at 03:06:01PM -0800, Mark Millard wrote:
> 
> One thing that could fit the behavior is if small part(s)
> of the system c++ compiler (or libraires it uses) were
> corrupted on that specific media. In that case, nothing
> elsewhere would replicate the failures but a lot might
> work without using the corrupted part(s), making the
> failures not random. 

[spaced for emphasis]

> Checking on that is part of why
> I'd hoped to get a lldb report for a .sh/.cpp pair
> leading to failure on your RPi3* in question.
>

If/when the stable/13 Pi3 finishes its -j1 single-user
build/install cycle I'll make a point of trying the 
.sh/.cpp test under lldb.  

For most of their operational history both troublesome Pi3
systems have had some of their swap on microSD. If there
is no error detection at all for microSD-based storage
then undetected corruption of data from swap is a real
possibility. I expected that storage errors would be
reported but maybe not, especially outside file systems.  

Mechanical disks have some internal error detection and
report explictly when data can't be retrieved. As I think
back on it at least one flash device (a USB thumb drive)
failed silently, no reported errors but also no-write.
That was on a filesystem, so the OS noticed and so did I.

Is there any error detection/correction employed by the
virtual memory system as it reads and writes mass storage? 

Thanks for reading!