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Miller" , linux-sparc Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends In-Reply-To: References: <20200919091751.011116649@linutronix.de> <87mu1lc5mp.fsf@nanos.tec.linutronix.de> <87k0wode9a.fsf@nanos.tec.linutronix.de> <87eemwcpnq.fsf@nanos.tec.linutronix.de> Date: Mon, 21 Sep 2020 09:39:39 +0200 Message-ID: <87a6xjd1dw.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain 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 Sun, Sep 20 2020 at 10:42, Linus Torvalds wrote: > On Sun, Sep 20, 2020 at 10:40 AM Thomas Gleixner wrote: >> >> I think the more obvious solution is to split the whole exercise: >> >> schedule() >> prepare_switch() >> unmap() >> >> switch_to() >> >> finish_switch() >> map() > > Yeah, that looks much easier to explain. Ack. So far so good, but Peter Z. just pointed out to me that I completely missed the fact that this cannot work. If a task is migrated to a different CPU then the mapping address will change which will explode in colourful ways. On RT kernels this works because we ping the task to the CPU via migrate_disable(). On a !RT kernel migrate_disable() maps to preempt_disable() which brings us back to square one. /me goes back to the drawing board. Thanks, tglx