From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx120.postini.com [74.125.245.120]) by kanga.kvack.org (Postfix) with SMTP id B2A936B0070 for ; Thu, 14 Jun 2012 10:28:53 -0400 (EDT) Message-ID: <4FD9F4EB.7080108@redhat.com> Date: Thu, 14 Jun 2012 10:27:55 -0400 From: Rik van Riel MIME-Version: 1.0 Subject: Re: bugs in page colouring code References: <20120613152936.363396d5@cuia.bos.redhat.com> <20120614132053.GD28714@n2100.arm.linux.org.uk> In-Reply-To: <20120614132053.GD28714@n2100.arm.linux.org.uk> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Russell King - ARM Linux Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, sjhill@mips.com, ralf@linux-mips.org, Borislav Petkov , "H. Peter Anvin" , Rob Herring , Nicolas Pitre On 06/14/2012 09:20 AM, Russell King - ARM Linux wrote: > On Wed, Jun 13, 2012 at 03:29:36PM -0400, Rik van Riel wrote: >> COLOUR_ALIGN_DOWN can use the pgoff % shm_align_mask either positively >> or negatively, depending on the address initially found by >> get_unmapped_area >> >> static inline unsigned long COLOUR_ALIGN_DOWN(unsigned long addr, >> unsigned long pgoff) >> { >> unsigned long base = addr& ~shm_align_mask; >> unsigned long off = (pgoff<< PAGE_SHIFT)& shm_align_mask; >> >> if (base + off<= addr) >> return base + off; >> >> return base - off; >> } > > Yes, that is bollocks code, introduced by this commit: > > commit 7dbaa466780a754154531b44c2086f6618cee3a8 > Author: Rob Herring > Date: Tue Nov 22 04:01:07 2011 +0100 It's not just ARM that has this bug. It appears to be cut'n'pasted from other architectures (MIPS? SPARC?). >> The fix would be to return an address that is a whole shm_align_mask >> lower: (((base - shm_align_mask)& ~shm_align_mask) + off > > Yes, agreed. I will make sure the arch-independent colouring code does that. > This brings up the question: should a MAP_PRIVATE mapping see updates > to the backing file made via a shared mapping and/or writing the file > directly? After all, a r/w MAP_PRIVATE mapping which has been CoW'd > won't see the updates. > > So I'd argue that a file mapped MAP_SHARED must be mapped according to > the colour rules, but a MAP_PRIVATE is free not to be so. OK, fair enough. >> Secondly, MAP_FIXED never checks for page colouring alignment. >> I assume the cache aliasing on AMD Bulldozer is merely a performance >> issue, and we can simply ignore page colouring for MAP_FIXED? >> That will be easy to get right in an architecture-independent >> implementation. > > There's a whole bunch of issues with MAP_FIXED, specifically address > space overflow has been discussed previously, and resulted in this patch: > > [PATCH 0/6] get rid of extra check for TASK_SIZE in get_unmapped_area Turns out, get_unmapped_area_prot (the function that calls arch_get_unmapped_area) checks for these overflows, so we should be fine. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org