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[209.85.208.50]) by smtp.gmail.com with ESMTPSA id g1sm2485537ejz.52.2020.10.30.15.53.19 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 30 Oct 2020 15:53:19 -0700 (PDT) Received: by mail-ed1-f50.google.com with SMTP id p93so8284006edd.7 for ; Fri, 30 Oct 2020 15:53:19 -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 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201030_185321_397794_042D965C X-CRM114-Status: GOOD ( 18.23 ) X-BeenThere: linux-snps-arc@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on Synopsys ARC Processors List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Juri Lelli , linux-xtensa@linux-xtensa.org, Peter Zijlstra , Benjamin Herrenschmidt , Sebastian Andrzej Siewior , linux-mips@vger.kernel.org, Ben Segall , Linux-MM , Guo Ren , Matthew Wilcox , linux-sparc , Vincent Chen , Ingo Molnar , linux-arch , Vincent Guittot , Herbert Xu , Michael Ellerman , the arch/x86 maintainers , Russell King , linux-csky@vger.kernel.org, Christoph Hellwig , David Airlie , Mel Gorman , "open list:SYNOPSYS ARC ARCHITECTURE" , Ard Biesheuvel , Paul McKenney , linuxppc-dev , Steven Rostedt , Greentime Hu , Dietmar Eggemann , Linux ARM , Chris Zankel , Michal Simek , Thomas Bogendoerfer , Nick Hu , Max Filippov , Vineet Gupta , LKML , Arnd Bergmann , Daniel Vetter , Paul Mackerras , Andrew Morton , Daniel Bristot de Oliveira , "David S. Miller" Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-snps-arc" Errors-To: linux-snps-arc-bounces+linux-snps-arc=archiver.kernel.org@lists.infradead.org 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 _______________________________________________ linux-snps-arc mailing list linux-snps-arc@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-snps-arc