Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chainsupport to use NMI-safe methods

[Mathieu Desnoyers]

* H. Peter Anvin (hpa@xxxxxxxxx) wrote:
> Mathieu Desnoyers wrote:
> > 
> > Where is the kernel page fault handler grabbing any lock to service
> > in-kernel page faults exactly ?
> > 
> > Those are usually considered as utterly simple page table fixups,
> > nothing more.
> > 
> 
> It doesn't, *because it doesn't have to*.  Your proposal requires that
> page faults can be handled inside the page fault handler, and that's a
> pretty tall order.
> 
> 	-hpa
> 

I am not asking for the pf handler to handle every possible kind of
fault recursively. Just to keep the in-kernel page fault related code
for vmalloc (and possibly for prefetch ?) paths NMI-reentrant :

void do_page_fault(struct pt_regs *regs, unsigned long error_code)

  address = read_cr2();

  if (unlikely(kmmio_fault(regs, address)))
    return;

#ifdef CONFIG_X86_32
        if (unlikely(address >= TASK_SIZE)) {
#else
        if (unlikely(address >= TASK_SIZE64)) {
#endif
                if (!(error_code & (PF_RSVD|PF_USER|PF_PROT)) &&
                    vmalloc_fault(address) >= 0)
                        return;

                /* Can handle a stale RO->RW TLB */
                if (spurious_fault(address, error_code))
                        return;

                /* kprobes don't want to hook the spurious faults. */
                if (notify_page_fault(regs))
                        return;
                /*
                 * Don't take the mm semaphore here. If we fixup a prefetch
                 * fault we could otherwise deadlock.
                 */
                goto bad_area_nosemaphore;
        }

This includes vmalloc_fault. Note that I already looked into
vmalloc_fault to ensure it could handle NMIs on x86_64 as well. See
commit bdd5ea31e79fed76eb57d0cd797355267f4f4a8c. It seems I missed the
cr2 register issue though. I assumed NMI handler to save it somehow,
which ends up not being the case.

Mathieu




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Mathieu Desnoyers
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