Re: why do we need vmalloc_sync_all?

[Andy Lutomirski]

On Sun, Jun 14, 2015 at 7:47 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> Oleg Nesterov <oleg@xxxxxxxxxx> writes:
>>
>> 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.
>
> The same problem can happen from NMI handlers or machine check
> handlers. It's not necessarily tied to page faults only.

AIUI, the point of the one and only vmalloc_sync_all call is to
prevent infinitely recursive faults when we call a notify_die
callback.  The only thing that it could realistically protect is
module text or static non-per-cpu module data, since that's the only
thing that's reliably already in the init pgd.  I'm with Oleg: I don't
see how that can happen, since do_page_fault fixes up vmalloc faults
before it calls notify_die.

--Andy
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