From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH v3 1/2] mm/highmem: make kmap cache coloring aware Date: Fri, 1 Aug 2014 13:10:49 -0700 Message-ID: <20140801131049.e94e0e6daec0180ac0236f68@linux-foundation.org> References: <1406317427-10215-1-git-send-email-jcmvbkbc@gmail.com> <1406317427-10215-2-git-send-email-jcmvbkbc@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: Received: from mail.linuxfoundation.org ([140.211.169.12]:36962 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754697AbaHAUKv (ORCPT ); Fri, 1 Aug 2014 16:10:51 -0400 In-Reply-To: <1406317427-10215-2-git-send-email-jcmvbkbc@gmail.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Max Filippov Cc: linux-xtensa@linux-xtensa.org, Chris Zankel , Marc Gauthier , linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org, David Rientjes , Leonid Yegoshin , Steven Hill On Fri, 25 Jul 2014 23:43:46 +0400 Max Filippov wrote: > VIPT cache with way size larger than MMU page size may suffer from > aliasing problem: a single physical address accessed via different > virtual addresses may end up in multiple locations in the cache. > Virtual mappings of a physical address that always get cached in > different cache locations are said to have different colors. > L1 caching hardware usually doesn't handle this situation leaving it > up to software. Software must avoid this situation as it leads to > data corruption. > > One way to handle this is to flush and invalidate data cache every time > page mapping changes color. The other way is to always map physical page > at a virtual address with the same color. Low memory pages already have > this property. Giving architecture a way to control color of high memory > page mapping allows reusing of existing low memory cache alias handling > code. > > Provide hooks that allow architectures with aliasing cache to align > mapping address of high pages according to their color. Such architectures > may enforce similar coloring of low- and high-memory page mappings and > reuse existing cache management functions to support highmem. > > This code is based on the implementation of similar feature for MIPS by > Leonid Yegoshin . > It's worth mentioning that xtensa needs this. What is (still) missing from these changelogs is a clear description of the end-user visible effects. Does it fix some bug? If so what? Is it a performace optimisation? If so how much? This info is the top-line reason for the patchset and should be presented as such. > --- a/mm/highmem.c > +++ b/mm/highmem.c > @@ -28,6 +28,9 @@ > #include > #include > #include > +#ifdef CONFIG_HIGHMEM > +#include > +#endif Should be unneeded - the linux/highmem.h inclusion already did this. Apart from that it all looks OK to me. I'm assuming this is 3.17-rc1 material, but I am unsure because of the missing end-user-impact info. If it's needed in earlier kernels then we can tag it for -stable backporting but again, the -stable team (ie: Greg) will want so see the justification for that backport.