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Date:   Mon, 02 Nov 2020 02:08:23 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     linux-arch@...r.kernel.org,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Ard Biesheuvel <ardb@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Christoph Hellwig <hch@....de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        x86@...nel.org, Vineet Gupta <vgupta@...opsys.com>,
        linux-snps-arc@...ts.infradead.org,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org, Guo Ren <guoren@...nel.org>,
        linux-csky@...r.kernel.org, Michal Simek <monstr@...str.eu>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org, Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org,
        "David S. Miller" <davem@...emloft.net>,
        sparclinux@...r.kernel.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends

On Thu, Oct 29 2020 at 23:18, Thomas Gleixner wrote:
>
> There is also a still to be investigated question from Linus on the initial
> posting versus the per cpu / per task mapping stack depth which might need
> to be made larger due to the ability to take page faults within a mapping
> region.

I looked deeper into that and we have a stack depth of 20. That's plenty
and I couldn't find a way to get above 10 nested ones including faults,
interrupts, softirqs. With some stress testing I was not able to get over
a maximum of 6 according to the traceprintk I added.

For some obscure reason when CONFIG_DEBUG_HIGHMEM is enabled the stack
depth is increased from 20 to 41. But the only thing DEBUG_HIGHMEM does
is to enable a few BUG_ON()'s in the mapping code.

That's a leftover from the historical mapping code which had fixed
entries for various purposes. DEBUG_HIGHMEM inserted guard mappings
between the map types. But that got all ditched when kmap_atomic()
switched to a stack based map management. Though the WITH_KM_FENCE magic
survived without being functional. All the thing does today is to
increase the stack depth.

I just made that functional again by keeping the stack depth increase
and utilizing every second slot. That should catch Willy's mapping
problem nicely if he bothers to test on 32bit :)

Thanks,

        tglx

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