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[209.85.218.42]) by smtp.gmail.com with ESMTPSA id 6sm3471055ejv.49.2020.10.30.15.54.03 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 30 Oct 2020 15:54:03 -0700 (PDT) Received: by mail-ej1-f42.google.com with SMTP id s15so10736340ejf.8 for ; Fri, 30 Oct 2020 15:54:03 -0700 (PDT) X-Received: by 2002:a19:4815:: with SMTP id v21mr1949766lfa.603.1604098002560; Fri, 30 Oct 2020 15:46:42 -0700 (PDT) MIME-Version: 1.0 References: <20201029221806.189523375@linutronix.de> <87pn50ob0s.fsf@nanos.tec.linutronix.de> <87blgknjcw.fsf@nanos.tec.linutronix.de> <87sg9vl59i.fsf@nanos.tec.linutronix.de> In-Reply-To: <87sg9vl59i.fsf@nanos.tec.linutronix.de> From: Linus Torvalds Date: Fri, 30 Oct 2020 15:46:26 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends To: Thomas Gleixner Cc: LKML , linux-arch , Peter Zijlstra , Paul McKenney , David Airlie , Daniel Vetter , Ard Biesheuvel , Herbert Xu , Christoph Hellwig , Sebastian Andrzej Siewior , Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Andrew Morton , Linux-MM , "the arch/x86 maintainers" , Vineet Gupta , "open list:SYNOPSYS ARC ARCHITECTURE" , Russell King , Arnd Bergmann , Linux ARM , Guo Ren , linux-csky@vger.kernel.org, Michal Simek , Thomas Bogendoerfer , linux-mips@vger.kernel.org, Nick Hu , Greentime Hu , Vincent Chen , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , linuxppc-dev , "David S. Miller" , linux-sparc , Chris Zankel , Max Filippov , linux-xtensa@linux-xtensa.org, Matthew Wilcox Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Oct 30, 2020 at 3:26 PM Thomas Gleixner wrote: > > While at it I might have a look at that debug request from Willy in the > other end of this thread. Any comment on that? > > https://lore.kernel.org/r/87k0v7mrrd.fsf@nanos.tec.linutronix.de I do think that it would be nice to have a debug mode, particularly since over the last few years we've really lost a lot of HIGHMEM coverage (to the point that I've wondered how worthwhile it really is to support at all any more - I think it's Arnd who argued that it's mainly some embedded ARM variants that will want it for the forseeable future). So I'm honestly somewhat torn. I think HIGHMEM is dying, and yes that argues for "non-HIGHMEM had better have some debug coverage", but at the same time I think it doesn't even really matter any more. At some point those embedded ARM platforms just aren't even interesting - they might as well use older kernels if they are the only thing really arguing for HIGHMEM at all. This is one reason why I'd like the new kmap_local() to be a no-op, and I'd prefer for it to have no other side effects - because I want to be ready to remove it entirely some day. And if we end up having some transition where people start rewriting "kmap_atomic()" to be "kmap_local() + explicit preemption disable", then I think that would be a good step on that whole "kmap will eventually go away" path. But I do *not* believe that we need to add _so_ much debug support that we'd catch Willy's "more than one page" case. And I absolutely do not believe for a second that we should start caring about compound pages. NO. kmap() is almost dead already, we're not making it worse. To me, your patch series has two big advantages: - more common code - kmap_local() becomes more of a no-op and the last thing we want is to expand on kmap. Linus