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From:   Thomas Gleixner <tglx@linutronix.de>
To:     peterz@infradead.org
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Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends
In-Reply-To: <20200923101953.GT2674@hirez.programming.kicks-ass.net>
References: <20200919091751.011116649@linutronix.de> <CAHk-=wiYGyrFRbA1cc71D2-nc5U9LM9jUJesXGqpPnB7E4X1YQ@mail.gmail.com> <87mu1lc5mp.fsf@nanos.tec.linutronix.de> <87k0wode9a.fsf@nanos.tec.linutronix.de> <CAHk-=wgbmwsTOKs23Z=71EBTrULoeaH2U3TNqT2atHEWvkBKdw@mail.gmail.com> <87eemwcpnq.fsf@nanos.tec.linutronix.de> <CAHk-=wgF-upZVpqJWK=TK7MS9H-Rp1ZxGfOG+dDW=JThtxAzVQ@mail.gmail.com> <87a6xjd1dw.fsf@nanos.tec.linutronix.de> <CAHk-=wjhxzx3KHHOMvdDj3Aw-_Mk5eRiNTUBB=tFf=vTkw1FeA@mail.gmail.com> <87sgbbaq0y.fsf@nanos.tec.linutronix.de> <20200923101953.GT2674@hirez.programming.kicks-ass.net>
Date:   Wed, 23 Sep 2020 14:33:17 +0200
Message-ID: <87sgb8ad0y.fsf@nanos.tec.linutronix.de>
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On Wed, Sep 23 2020 at 12:19, peterz wrote:
> On Mon, Sep 21, 2020 at 09:27:57PM +0200, Thomas Gleixner wrote:
>> Alternatively this could of course be solved with per CPU page tables
>> which will come around some day anyway I fear.
>
> Previously (with PTI) we looked at making the entire kernel map per-CPU,
> and that takes a 2K copy on switch_mm() (or more general, the user part
> of whatever the top level directory is for architectures that have a
> shared kernel/user page-table setup in the first place).
>
> The idea was having a fixed per-cpu kernel page-table, share a bunch of
> (kernel) page-tables between all CPUs and then copy in the user part on
> switch.
>
> I've forgotten what the plan was for ASID/PCID in that scheme.
>
> For x86_64 we've been fearing the performance of that 2k copy, but I
> don't think we've ever actually bit the bullet and implemented it to see
> how bad it really is.

I actually did at some point and depending on the workload the overhead
was clearly measurable. And yes, it fell apart with PCID and I could not
come up with a scheme for it which did not suck horribly. So I burried
the patches in the poison cabinet.

Aside of that, we'd need to implement that for a eight other
architectures as well...

Thanks,

        tglx