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Miller" , sparclinux@vger.kernel.org Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends Message-ID: <20200920082353.GG438822@phenom.ffwll.local> Mail-Followup-To: Thomas Gleixner , LKML , "open list:GENERIC INCLUDE/A..." , Linus Torvalds , Paul McKenney , X86 ML , 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 , intel-gfx , dri-devel , Ard Biesheuvel , Herbert Xu , Vineet Gupta , arcml , 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" , sparclinux@vger.kernel.org References: <20200919091751.011116649@linutronix.de> <87pn6hc6g1.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87pn6hc6g1.fsf@nanos.tec.linutronix.de> X-Operating-System: Linux phenom 5.7.0-1-amd64 Precedence: bulk List-ID: X-Mailing-List: linux-csky@vger.kernel.org On Sun, Sep 20, 2020 at 08:23:26AM +0200, Thomas Gleixner wrote: > On Sat, Sep 19 2020 at 12:37, Daniel Vetter wrote: > > On Sat, Sep 19, 2020 at 12:35 PM Daniel Vetter wrote: > >> I think it should be the case, but I want to double check: Will > >> copy_*_user be allowed within a kmap_temporary section? This would > >> allow us to ditch an absolute pile of slowpaths. > > > > (coffee just kicked in) copy_*_user is ofc allowed, but if you hit a > > page fault you get a short read/write. This looks like it would remove > > the need to handle these in a slowpath, since page faults can now be > > served in this new kmap_temporary sections. But this sounds too good > > to be true, so I'm wondering what I'm missing. > > In principle we could allow pagefaults, but not with the currently > proposed interface which can be called from any context. Obviously if > called from atomic context it can't handle user page faults. Yeah that's clear, but does the implemention need to disable pagefaults unconditionally? > In theory we could make a variant which does not disable pagefaults, but > that's what kmap() already provides. Currently we have a bunch of code which roughly does kmap_atomic(); copy_*_user(); kunmap_atomic(); if (short_copy_user) { kmap(); copy_*_user(remaining_stuff); kunmap(); } And the only reason is that kmap is a notch slower, hence the fastpath. If we get a kmap which is fast and allows pagefaults (only in contexts that allow pagefaults already ofc) then we can ditch a pile of kmap users. Cheers, Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch