Messages in this thread | | | Date | Sun, 14 Jun 2015 22:06:23 +0200 | From | Oleg Nesterov <> | Subject | Re: why do we need vmalloc_sync_all? |
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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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