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SubjectRe: [PATCH RESEND v6 00/16] mm: Page fault enhancements
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[...]

> Yes, IIUC the race can happen like this in your below test:
>
> main thread uffd thread disgard thread
> =========== =========== ==============
> access page
> uffd page fault
> wait for page
> UFFDIO_ZEROCOPY
> put a page P there
> MADV_DONTNEED on P
> wakeup main thread
> return from fault
> page still missing
> uffd page fault again
> (without ALLOW_RETRY)
> --> SIGBUS.

Exactly!

>> Can we please have a way to identify that this "feature" is available?
>> I'd appreciate a new read-only UFFD_FEAT_ , so we can detect this from
>> user space easily and use concurrent discards without crashing our applications.
>
> I'm not sure how others think about it, but to me this still fells
> into the bucket of "solving an existing problem" rather than a
> feature. Also note that this should change the behavior for the page
> fault logic in general, rather than an uffd-only change. So I'm also
> not sure whether UFFD_FEAT_* suites here even if we want it.

So, are we planning on backporting this to stable kernels?

Imagine using this in QEMU/KVM to allow discards (e.g., balloon
inflation) while postcopy is active . You certainly don't want random
guest crashes. So either, we treat this as a fix (and backport) or as a
change in behavior/feature.

[...]

>>
>> 2. What will happen if I don't place a page on a pagefault, but only do a UFFDIO_WAKE?
>> For now we were able to trigger a signal this way.
>
> If I'm not mistaken the UFFDIO_WAKE will directly trigger the sigbus
> even without the help of the MADV_DONTNEED race.

Yes, that's the current way of injecting a SIGBUS instead of resolving
the pagefault. And AFAIKs, you're changing that behavior. (I am not
aware of a user, there could be use cases, but somehow it's strange to
get a signal when accessing memory that is mapped READ|WRITE and also
represented like this in e.g., /proc/$PID/maps). So IMHO, only the new
behavior makes really sense.

>
>> If the behavior is changed, can
>> we make this configurable via a UFFD_FEAT?
>
> I'll still think that could be an overkill, but I'll leave the
> discussion to the experts.

I'll be happy to hear what Andrea Et al. think. At least I really want
to see the new behavior - and if it's not a fix, then I want some way to
detect if a kernel has this new (fixed?) behavior.

Thanks a lot for this work!

--
Thanks,

David / dhildenb

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