David, do you feel that the proposed fix will at least address the bug
without adverse side-effects?
Usually, when I suspect something is dodgy I unconsciously push back
harder than I usually would.
Please consider using unconsciousness only for self guidance, figuring out
directions, or making decisions on one's own.
Yeah, sorry about my communication. I expressed that this approach felt
wrong to me, I just wasn't able to phrase exactly why I thought
migration is doing the right thing and didn't have a lot of time to look
into the details.
Now I dedicated some time and realized that mproctect() is doing the
exact same thing, it became clearer to me why migration code wasn't
broken before.
For discussions on the list which can get more than one person involved, we
do need consciousness and reasonings.
Yeah, I need vacation.
Thanks for the reproducer, that's definitely good reasonings. Do you have
other reproducer that can trigger an issue without mprotect()?
As noted in the RFC patch I sent, I suspect NUMA hinting page remapping
might similarly trigger it. I did not try reproducing it, though.
As I probably mentioned before in other threads mprotect() is IMHO
conceptually against uffd-wp and I don't yet figured out how to use them
all right. For example, we can uffd-wr-protect a pte in uffd-wp range,
then if we do "mprotect(RW)" it's hard to tell whether the user wants it
write or not. E.g., using mprotect(RW) to resolve page faults should be
wrong because it'll not touch the uffd-wp bit at all. I confess I never
thought more on how we should define the interactions between uffd-wp and
mprotect.
In short, it'll be great if you have other reproducers for any uffd-wp
issues other than mprotect().
I said that also because I just got another message from Ives privately
that there _seems_ to have yet another even harder to reproduce bug here
(Ives, feel free to fill in any more information if you got it). So if you
can figure out what's missing and already write a reproducer, that'll be
perfect.
Maybe NUMA hitning on the fallback path, when we didn't migrate or
migration failed?
--
Thanks,
David / dhildenb
