Re: update_mmu_cache(): fault or not fault ?

["David S. Miller" <davem@davemloft.net>]

From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Mon, 26 Sep 2005 16:22:05 +1000

> The problem is that want to only ever do that kind of hw TLB pre-fill
> when update_mmu_cache() is called as the result an actual fault.
> However, for some reasons that I'm not 100% sure about (*)
> update_mmu_cache() is called from other places, typically in mm/fremap.c
> which aren't directly results of faults.
> 
> So I suggest adding an argument to it "int is_fault", that would
> basically be '1' on all the call sites in mm/memory.c and '0' in all the
> call sites in mm/fremap.c.

You can track this in your port specific code.  That's what I do on
sparc64 to deal with this case.  I record the TLB miss type (D or I
tlb), and also whether a write occurred, in a bitmask.  Then I check
this in update_mmu_cache() to decide whether to prefill.

I store it in current_thread_info() and clear it at the end of fault
processing.

Just grep for "FAULT_CODE_*" in the sparc64 code to see how this
works.

Although, I'm ambivalent as to whether prefilling helps at all.
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