Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934131AbaGPA3E (ORCPT ); Tue, 15 Jul 2014 20:29:04 -0400 Received: from mail-la0-f46.google.com ([209.85.215.46]:55496 "EHLO mail-la0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754788AbaGPA3B (ORCPT ); Tue, 15 Jul 2014 20:29:01 -0400 MIME-Version: 1.0 In-Reply-To: <1405468387.28702.53.camel@misato.fc.hp.com> References: <1405452884-25688-1-git-send-email-toshi.kani@hp.com> <1405452884-25688-4-git-send-email-toshi.kani@hp.com> <1405465801.28702.34.camel@misato.fc.hp.com> <1405468387.28702.53.camel@misato.fc.hp.com> From: Andy Lutomirski Date: Tue, 15 Jul 2014 17:28:40 -0700 Message-ID: Subject: Re: [RFC PATCH 3/11] x86, mm, pat: Change reserve_memtype() to handle WT type To: Toshi Kani Cc: "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , Andrew Morton , Arnd Bergmann , Konrad Rzeszutek Wilk , plagnioj@jcrosoft.com, tomi.valkeinen@ti.com, "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Stefan Bader , Dave Airlie , Borislav Petkov Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 15, 2014 at 4:53 PM, Toshi Kani wrote: > On Tue, 2014-07-15 at 16:36 -0700, Andy Lutomirski wrote: >> On Tue, Jul 15, 2014 at 4:10 PM, Toshi Kani wrote: >> > On Tue, 2014-07-15 at 12:56 -0700, Andy Lutomirski wrote: >> >> On Tue, Jul 15, 2014 at 12:34 PM, Toshi Kani wrote: >> >> > This patch changes reserve_memtype() to handle the new WT type. >> >> > When (!pat_enabled && new_type), it continues to set either WB >> >> > or UC- to *new_type. When pat_enabled, it can reserve a given >> >> > non-RAM range for WT. At this point, it may not reserve a RAM >> >> > range for WT since reserve_ram_pages_type() uses the page flags >> >> > limited to three memory types, WB, WC and UC. >> >> >> >> FWIW, last time I looked at this, it seemed like all the fancy >> >> reserve_ram_pages stuff was unnecessary: shouldn't the RAM type be >> >> easy to track in the direct map page tables? >> > >> > Are you referring the direct map page tables as the kernel page >> > directory tables (pgd/pud/..)? >> > >> > I think it needs to be able to keep track of the memory type per a >> > physical memory range, not per a translation, in order to prevent >> > aliasing of the memory type. >> >> Actual RAM (the lowmem kind, which is all of it on x86_64) is mapped >> linearly somewhere in kernel address space. The pagetables for that >> mapping could be used as the canonical source of the memory type for >> the ram range in question. >> >> This only works for lowmem, so maybe it's not a good idea to rely on it. > > Right. > > I think using struct page table for the RAM ranges is a good way for > saving memory, but I wonder how often the RAM ranges are mapped other > than WB... If not often, reserve_memtype() could simply call > rbt_memtype_check_insert() for all ranges, including RAM. > > In this patch, I left using reserve_ram_pages_type() since I do not see > much reason to use WT for RAM, either. I hereby predict that someone, some day, will build a system with nonvolatile "RAM", and someone will want this feature. Just saying :) More realistically, someone might want to write a silly driver that lets programs mmap some WT memory for testing. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/