Re: linux-next: manual merge of the akpm-current tree with the tip tree

[Minchan Kim <minchan@kernel.org>]

On Sun, Aug 13, 2017 at 02:50:19PM +0200, Peter Zijlstra wrote:
> On Sun, Aug 13, 2017 at 06:06:32AM +0000, Nadav Amit wrote:
> > > however mm_tlb_flush_nested() is a mystery, it appears to care about
> > > anything inside the range. For now rely on it doing at least _a_ PTL
> > > lock instead of taking  _the_ PTL lock.
> > 
> > It does not care about “anything” inside the range, but only on situations
> > in which there is at least one (same) PT that was modified by one core and
> > then read by the other. So, yes, it will always be _the_ same PTL, and not
> > _a_ PTL - in the cases that flush is really needed.
> > 
> > The issue that might require additional barriers is that
> > inc_tlb_flush_pending() and mm_tlb_flush_nested() are called when the PTL is
> > not held. IIUC, since the release-acquire might not behave as a full memory
> > barrier, this requires an explicit memory barrier.
> 
> So I'm not entirely clear about this yet.
> 
> How about:
> 
> 
> 	CPU0				CPU1
> 
> 					tlb_gather_mmu()
> 
> 					lock PTLn
> 					no mod
> 					unlock PTLn
> 
> 	tlb_gather_mmu()
> 
> 					lock PTLm
> 					mod
> 					include in tlb range
> 					unlock PTLm
> 
> 	lock PTLn
> 	mod
> 	unlock PTLn
> 
> 					tlb_finish_mmu()
> 					  force = mm_tlb_flush_nested(tlb->mm);
> 					  arch_tlb_finish_mmu(force);
> 
> 
> 	... more ...
> 
> 	tlb_finish_mmu()
> 
> 
> 
> In this case you also want CPU1's mm_tlb_flush_nested() call to return
> true, right?

No, because CPU 1 mofified pte and added it into tlb range
so regardless of nested, it will flush TLB so there is no stale
TLB problem.

> 
> But even with an smp_mb__after_atomic() at CPU0's tlg_bather_mmu()
> you're not guaranteed CPU1 sees the increment. The only way to do that
> is to make the PTL locks RCsc and that is a much more expensive
> proposition.
> 
> 
> What about:
> 
> 
> 	CPU0				CPU1
> 
> 					tlb_gather_mmu()
> 
> 					lock PTLn
> 					no mod
> 					unlock PTLn
> 
> 
> 					lock PTLm
> 					mod
> 					include in tlb range
> 					unlock PTLm
> 
> 	tlb_gather_mmu()
> 
> 	lock PTLn
> 	mod
> 	unlock PTLn
> 
> 					tlb_finish_mmu()
> 					  force = mm_tlb_flush_nested(tlb->mm);
> 					  arch_tlb_finish_mmu(force);
> 
> 
> 	... more ...
> 
> 	tlb_finish_mmu()
> 
> Do we want CPU1 to see it here? If so, where does it end?

Ditto. Since CPU 1 has added range, it will flush TLB regardless
of nested condition.

> 
> 	CPU0				CPU1
> 
> 					tlb_gather_mmu()
> 
> 					lock PTLn
> 					no mod
> 					unlock PTLn
> 
> 
> 					lock PTLm
> 					mod
> 					include in tlb range
> 					unlock PTLm
> 
> 					tlb_finish_mmu()
> 					  force = mm_tlb_flush_nested(tlb->mm);
> 
> 	tlb_gather_mmu()
> 
> 	lock PTLn
> 	mod
> 	unlock PTLn
> 
> 					  arch_tlb_finish_mmu(force);
> 
> 
> 	... more ...
> 
> 	tlb_finish_mmu()
> 
> 
> This?
> 
> 
> Could you clarify under what exact condition mm_tlb_flush_nested() must
> return true?

mm_tlb_flush_nested aims for the CPU side where there is no pte update
but need TLB flush.
As I wrote https://marc.info/?l=linux-mm&m=150267398226529&w=2,
it has stable TLB problem if we don't flush TLB although there is no
pte modification.