From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Wilson Date: Thu, 10 Mar 2011 16:40:22 +0000 Subject: Re: [PATCH 0/5] make *_gate_vma accept mm_struct instead of Message-Id: <20110310164022.GA6242@fibrous.localdomain> List-Id: References: <1299630721-4337-1-git-send-email-wilsons@start.ca> <20110310160032.GA20504@alboin.amr.corp.intel.com> In-Reply-To: <20110310160032.GA20504@alboin.amr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Andi Kleen Cc: x86@kernel.org, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Benjamin Herrenschmidt , Paul Mackerras , Martin Schwidefsky , Heiko Carstens , linux390@de.ibm.com, Paul Mundt , Michel Lespinasse , Andrew Morton , Alexander Viro , linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org 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