Re: why do we need vmalloc_sync_all?

[Oleg Nesterov <oleg@redhat.com>]

On 06/14, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@redhat.com> wrote:
>
> > I didn't read v2 yet, but I'd like to ask a question.
> >
> > Why do we need vmalloc_sync_all()?
> >
> > It has a single caller, register_die_notifier() which calls it without
> > any explanation. IMO, this needs a comment at least.
>
> Yes, it's used to work around crashes in modular callbacks: if the callbacks
> happens to be called from within the page fault path, before the vmalloc page
> fault handler runs, then we have a catch-22 problem.
>
> It's rare but not entirely impossible.

But again, the kernel no longer does this? do_page_fault() does vmalloc_fault()
without notify_die(). If it fails, I do not see how/why a modular DIE_OOPS
handler could try to resolve this problem and trigger another fault.

> > I am not sure I understand the changelog in 101f12af correctly, but at first
> > glance vmalloc_sync_all() is no longer needed at least on x86, do_page_fault()
> > no longer does notify_die(DIE_PAGE_FAULT). And btw DIE_PAGE_FAULT has no users.
> > DIE_MNI too...
> >
> > Perhaps we can simply kill it on x86?
>
> So in theory we could still have it run from DIE_OOPS, and that could turn a
> survivable kernel crash into a non-survivable one.

I don't understand... But OK, my understanding of this magic is very limited,
please forget.

Thanks,

Oleg.

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