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[209.85.167.47]) by smtp.gmail.com with ESMTPSA id r77sm1928471lff.115.2020.09.20.10.59.06 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 20 Sep 2020 10:59:06 -0700 (PDT) Received: by mail-lf1-f47.google.com with SMTP id b22so11501972lfs.13 for ; Sun, 20 Sep 2020 10:59:06 -0700 (PDT) X-Received: by 2002:ac2:4ec7:: with SMTP id p7mr12591149lfr.352.1600624746105; Sun, 20 Sep 2020 10:59:06 -0700 (PDT) MIME-Version: 1.0 References: <20200919091751.011116649@linutronix.de> <87mu1lc5mp.fsf@nanos.tec.linutronix.de> <87k0wode9a.fsf@nanos.tec.linutronix.de> <87eemwcpnq.fsf@nanos.tec.linutronix.de> In-Reply-To: From: Linus Torvalds Date: Sun, 20 Sep 2020 10:58:50 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends To: Thomas Gleixner Cc: LKML , linux-arch , Paul McKenney , "the arch/x86 maintainers" , Sebastian Andrzej Siewior , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Will Deacon , Andrew Morton , Linux-MM , Russell King , Linux ARM , Chris Zankel , Max Filippov , linux-xtensa@linux-xtensa.org, Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , David Airlie , Daniel Vetter , intel-gfx , dri-devel , Ard Biesheuvel , Herbert Xu , Vineet Gupta , "open list:SYNOPSYS ARC ARCHITECTURE" , Arnd Bergmann , 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Sep 20, 2020 at 10:42 AM Linus Torvalds wrote: > > Yeah, that looks much easier to explain. Ack. Btw, one thing that might be a good idea at least initially is to add a check for p->kmap_ctrl.idx being zero at fork, exit and maybe syscall return time (but that last one may be too cumbersome to really worry about). The kmap_atomic() interface basically has a lot of coverage for leaked as part of all the "might_sleep()" checks sprinkled around, The new kmap_temporary/local/whatever wouldn't have that kind of incidental debug checking, and any leaked kmap indexes would be rather hard to debug (much) later when they cause index overflows or whatever. Linus