* Oleg Nesterov <[email protected]> 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.
> 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.
Note that all of this will go away if we also do the vmalloc fault handling
simplification that I discussed with Andy:
- this series already makes the set of kernel PGDs strictly monotonically
increasing during the lifetime of the x86 kernel
- if in a subsequent patch we can synchronize new PGDs right after the vmalloc
code creates it, before the area is used - so we can remove vmalloc_fault()
altogether [or rather, turn it into a debug warning initially].
vmalloc_fault() is a clever but somewhat fragile complication.
- after that we can simply remove vmalloc_sync_all() from x86, because all
active
vmalloc areas will be fully instantiated, all the time, on x86.
Thanks,
Ingo
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