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Subject: Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for
 RCU_TABLE_FREE
From:   Benjamin Herrenschmidt <benh@kernel.crashing.org>
To:     Nicholas Piggin <npiggin@gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will.deacon@arm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        David Miller <davem@davemloft.net>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>
Date:   Tue, 28 Aug 2018 08:13:45 +1000
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On Mon, 2018-08-27 at 19:02 +1000, Nicholas Piggin wrote:
> > More tlbies ? With the cost of the broadasts on the fabric ? I don't
> > think so.. or I'm not understanding your point...
> 
> More tlbies are no good, but there will be some places where it works
> out much better (and fewer tlbies). Worst possible case for current code
> is a big unmap with lots of scattered page sizes. We _should_ get that
> with just a single PID flush at the end, but what we will get today is
> a bunch of PID and VA flushes.
> 
> I don't propose doing that though, I'd rather be explicit about
> tracking start and end range of each page size. Still not "optimal"
> but neither is existing single range for sparse mappings... anyway it
> will need to be profiled, but my point is we don't really fit exactly
> what x86/arm want.

If we have an arch specific part, we could just remember up to N
"large" pages there without actually flushing, and if that overflows,
upgrade to a full flush.

Cheers,
Ben.