linux-kernel - Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush

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Date:   Wed, 2 Aug 2017 10:02:28 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        torvalds@...ux-foundation.org, oleg@...hat.com,
        paulmck@...ux.vnet.ibm.com, mpe@...erman.id.au, npiggin@...il.com,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        stern@...land.harvard.edu, Mel Gorman <mgorman@...e.de>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush_pending()

On Wed, Aug 02, 2017 at 10:45:51AM +0200, Peter Zijlstra wrote:
> On Wed, Aug 02, 2017 at 09:15:23AM +0100, Will Deacon wrote:
> > On Wed, Aug 02, 2017 at 10:11:06AM +0200, Peter Zijlstra wrote:
> 
> > > arm64 looks good too, although it plays silly games with the first
> > > barrier, but I trust that to be sufficient.
> > 
> > The first barrier only orders prior stores for us, because page table
> > updates are made using stores. A prior load could be reordered past the
> > invalidation, but can't make it past the second barrier.
> 
> So then you rely on the program not having any loads pending to the
> address you're about to invalidate, right? Otherwise we can do the TLBI
> and then the load to insta-repopulate the TLB entry you just wanted
> dead.
> 
> That later DSB ISH is too late for that.
> 
> Isn't that somewhat fragile?

We only initiate the TLB invalidation after the page table update is
observable to the page table walker, so any repopulation will cause a fill
using the new page table entry.

> > I really think we should avoid defining TLB invalidation in terms of
> > smp_mb() because it's a lot more subtle than that.
> 
> I'm tempted to say stronger, smp_mb() only provides order, we want full
> serialization. Everything before stays before and _completes_ before.
> Everything after happens after (if the primitives actually do something
> at all of course, sparc64 for instance has no-op flush_tlb*).
> 
> While such semantics might be slightly too strong for what we currently
> need, it is what powerpc, x86 and arm currently implement and are fairly
> easy to reason about. If we weaken it, stuff gets confusing again.

My problem with this is that we're strengthening the semantics for no actual
use-case, but at the same time this will have a real performance impact.

Will