Re: tlb_gather_mmu() and semantics of "fullmm"

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2009-03-26 at 22:04 -0700, David Miller wrote:
> From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Date: Fri, 27 Mar 2009 09:33:44 +1100
> 
> > > I'd be surprised if there are still such optimizations to be made:
> > > maybe a whole different strategy could be more efficient, but I'd be
> > > surprised if there's really a superfluous TLB flush to be tweaked away.
> > > 
> > > Although it looks as if there's a TLB flush at the end of every batch,
> > > isn't that deceptive (on x86 anyway)?  I'm thinking that the first
> > > flush_tlb_mm() will end up calling leave_mm(), and the subsequent
> > > ones do nothing because the cpu_vm_mask is then empty.
> > 
> > Ok, well, that's a bit different on other archs like powerpc where we virtually
> > never remove bits from cpu_vm_mask... (though we probably could... to be looked
> > at).
> 
> We do this on sparc64 when the mm->mm_users == 1 and 'mm' is the
> current->active_mm

That doesn't sound right ... mm_user seems to represent how many tasks
have task->mm set to this mm, but now how many processors have it as
the "active_mm" due to lazy switching.

If you look at context_switch() in kernel/sched.c, it increments
mm_count when using the pevious guy's mm as the "active_mm" of a kernel
thread, not mm_user.

So effectively, mm_user can be any value, that doesn't represent how
many processors can have the mm currently active on them.

You could have mm_user be 1 due to the mm being active and in userspace
on another CPU, and have it locally be the active_mm because your local
CPU is in keventd or similar, flushing the other guy's mm as a result
of some unmap_mapping_range() call due to a network filesystem doing
coherency stuff for example.

Cheers,
Ben.

> See arch/sparc/kernel/smp_64.c:smp_flush_tlb_pending() where we go:
> 
> 	if (mm == current->active_mm && atomic_read(&mm->mm_users) == 1)
> 		mm->cpu_vm_mask = cpumask_of_cpu(cpu);
> 	else
> 		smp_cross_call_masked(&xcall_flush_tlb_pending,
> 				      ctx, nr, (unsigned long) vaddrs,
> 				      &mm->cpu_vm_mask);
> 
> 	__flush_tlb_pending(ctx, nr, vaddrs);

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