From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from terminus.zytor.com (terminus.zytor.com [198.137.202.10]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id 0F25D1007D1 for ; Fri, 11 Mar 2011 05:07:01 +1100 (EST) References: <1299630721-4337-1-git-send-email-wilsons@start.ca> <20110310160032.GA20504@alboin.amr.corp.intel.com> <20110310164022.GA6242@fibrous.localdomain> In-Reply-To: <20110310164022.GA6242@fibrous.localdomain> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----3X4K0B2TLOA62KIFLERUUQW17HJVJU" Subject: =?US-ASCII?Q?Re=3A_=5BPATCH_0/5=5D_make_*=5Fgate=5Fvma_acc?= =?US-ASCII?Q?ept_mm=5Fstruct_instead_of=09task=5Fstruct?= From: "H. Peter Anvin" Date: Thu, 10 Mar 2011 09:15:56 -0800 To: Stephen Wilson , Andi Kleen Message-ID: Cc: linux-s390@vger.kernel.org, Paul Mundt , linux-sh@vger.kernel.org, x86@kernel.org, Heiko Carstens , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Ingo Molnar , Paul Mackerras , Alexander Viro , Martin Schwidefsky , linux390@de.ibm.com, Thomas Gleixner , Michel Lespinasse , linuxppc-dev@lists.ozlabs.org, Andrew Morton List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , ------3X4K0B2TLOA62KIFLERUUQW17HJVJU Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit TIF_IA32 is set during the execution of a 32-bit system call - so touched on each compat system call. Is this the actual flag you want? A 32-bit address space flag is different from TIF_IA32. -- Sent from my mobile phone. Please pardon any lack of formatting. Stephen Wilson wrote: On Thu, Mar 10, 2011 at 08:00:32AM -0800, Andi Kleen wrote: > On Tue, Mar 08, 2011 at 07:31:56PM -0500, Stephen Wilson wrote: > > > > Morally, the question of whether an address lies in a gate vma should be asked > > with respect to an mm, not a particular task. > > > > Practically, dropping the dependency on task_struct will help make current and > > future operations on mm's more flexible and convenient. In particular, it > > allows some code paths to avoid the need to hold task_lock. > > > > The only architecture this change impacts in any significant way is x86_64. > > The principle change on that architecture is to mirror TIF_IA32 via > > a new flag in mm_context_t. > > The problem is -- you're adding a likely cache miss on mm_struct for > every 32bit compat syscall now, even if they don't need mm_struct > currently (and a lot of them do not) Unless there's a very good > justification to make up for this performance issue elsewhere > (including numbers) this seems like ! a bad idea. I do not think this will result in cache misses on the scale you suggest. I am simply mirroring the *state* of the TIF_IA32 flag in mm_struct, not testing/accessing it in the same way. The only place where this flag is accessed (outside the exec() syscall path) is in x86/mm/init_64.c, get_gate_vma(), which in turn is needed by a few, relatively heavy weight, page locking/pinning routines on the mm side (get_user_pages, for example). Patches 3 and 4 in the series show the extent of the change. Or am I missing something? > > /proc/pid/mem. I will be posting the second series to lkml shortly. These > > Making every syscall slower for /proc/pid/mem doesn't seem like a good > tradeoff to me. Please solve this in some other way. > > -Andi -- steve ------3X4K0B2TLOA62KIFLERUUQW17HJVJU Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: 8bit TIF_IA32 is set during the execution of a 32-bit system call - so touched on each compat system call. Is this the actual flag you want? A 32-bit address space flag is different from TIF_IA32.
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Sent from my mobile phone. Please pardon any lack of formatting.

Stephen Wilson <wilsons@start.ca> wrote:
On Thu, Mar 10, 2011 at 08:00:32AM -0800, Andi Kleen wrote: > On Tue, Mar 08, 2011 at 07:31:56PM -0500, Stephen Wilson wrote: > > > > Morally, the question of whether an address lies in a gate vma should be asked > > with respect to an mm, not a particular task. > > > > Practically, dropping the dependency on task_struct will help make current and > > future operations on mm's more flexible and convenient. In particular, it > > allows some code paths to avoid the need to hold task_lock. > > > > The only architecture this change impacts in any significant way is x86_64. > > The principle change on that architecture is to mirror TIF_IA32 via > > a new flag in mm_context_t. > > The problem is -- you're adding a likely cache miss on mm_struct for > every 32bit compat syscall now, even if they don't need mm_struct > currently (and a lot of them do not) Unless there's a very good > justification to make up for this performance issue elsewhere > (including numbers) this seems like a bad idea. I do not think this will result in cache misses on the scale you suggest. I am simply mirroring the *state* of the TIF_IA32 flag in mm_struct, not testing/accessing it in the same way. The only place where this flag is accessed (outside the exec() syscall path) is in x86/mm/init_64.c, get_gate_vma(), which in turn is needed by a few, relatively heavy weight, page locking/pinning routines on the mm side (get_user_pages, for example). Patches 3 and 4 in the series show the extent of the change. Or am I missing something? > > /proc/pid/mem. I will be posting the second series to lkml shortly. These > > Making every syscall slower for /proc/pid/mem doesn't seem like a good > tradeoff to me. Please solve this in some other way. > > -Andi -- steve
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