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Subject: Re: Potential race in TLB flush batching?
From: Nadav Amit <nadav.amit@gmail.com>
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Date: Wed, 12 Jul 2017 16:42:30 -0700
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Sender: owner-linux-mm@kvack.org
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To: Andy Lutomirski <luto@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>, "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>

Andy Lutomirski <luto@kernel.org> wrote:

> On Wed, Jul 12, 2017 at 4:27 PM, Nadav Amit <nadav.amit@gmail.com> =
wrote:
>> Actually, I think that based on Andy=E2=80=99s patches there is a =
relatively
>> reasonable solution. For each mm we will hold both a =
=E2=80=9Cpending_tlb_gen=E2=80=9D
>> (increased under the PT-lock) and an =E2=80=9Cexecuted_tlb_gen=E2=80=9D=
. Once
>> flush_tlb_mm_range finishes flushing it will use cmpxchg to update =
the
>> executed_tlb_gen to the pending_tlb_gen that was prior the flush (the
>> cmpxchg will ensure the TLB gen only goes forward). Then, whenever
>> pending_tlb_gen is different than executed_tlb_gen - a flush is =
needed.
>=20
> Why do we need executed_tlb_gen?  We already have
> cpu_tlbstate.ctxs[...].tlb_gen.  Or is the idea that executed_tlb_gen
> guarantees that all cpus in mm_cpumask are at least up to date to
> executed_tlb_gen?

Hm... So actually it may be enough, no? Just compare mm->context.tlb_gen
with cpu_tlbstate.ctxs[...].tlb_gen and flush if they are different?

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