Re: [PATCH v7 22/25] ACPI / APEI: Kick the memory_failure() queue for synchronous errors

[James Morse <james.morse@arm.com>]

Hi Boris,

On 22/01/2019 10:51, Borislav Petkov wrote:
> On Mon, Dec 10, 2018 at 07:15:13PM +0000, James Morse wrote:
>> What happens if we miss MF_ACTION_REQUIRED?
> 
> AFAICU, the logic is to force-send a signal to the user process, i.e.,
> force_sig_info() which cannot be ignored. IOW, an "enlightened" process
> would know how to do recovery action from a memory error.
> 
> VS the action optional thing which you can handle at your leisure.

> So the question boils down to what kind of severity do the errors
> reported through SEA have? I mean, if the hw would go the trouble to do
> the synchronous reporting, then something important must've happened and
> it wants us to know about it and handle it.

Before v8.2 we assumed these were fatal for the thread, it couldn't make progress.
Since v8.2 we get a value from the CPU, the severity values are, (the flippant
summary is obviously mine!):
* Recoverable: "You're about to step in it, fix it or die"
* Uncontainable: "It was here, but it escaped, we dont know where it went, panic!"
* Restartable/Corrected: "its fine, pretend this didn't happen"

Firmware should duplicate these values into the CPER severity fields.


>> Surely the page still gets unmapped as its PG_Poisoned, an AO signal
>> may be pending, but if user-space touches the page it will get an AR
>> signal. Is this just about removing an extra AO signal to user-space?

If we miss MF_ACTION_REQUIRED, the page still gets unmapped from user-space, and
user-space gets an AO signal. With this patch it takes that signal before it
continues. If it ignores it, the access gets a translation-fault->EHWPOISON->AR
signal from the arch code.

... so missing the flag gives us an extra signal. I'm not convinced this results
in any observable difference.


>> If we do need this, I'd like to pick it up from the CPER records, as x86's
>> NOTIFY_NMI looks like it covers both AO/AR cases. (as does NOTIFY_SDEI). The
>> Master/Target abort or Invalid-address types in the memory-error-section CPER
>> records look like the best bet.
> 
> Right, and we do all kinds of severity mapping there aka ghes_severity()
> so that'll be a good start, methinks.

The options are those 'aborts' in the memory error. These must have been a
result of some request. If we get a CPU error structure as part of the same
block, it may have a cache/bus error structure, which has a precise bit that
tells us whether this is a co-incidence. (but linux doesn't support any of those
structures today)



Thanks,

James