From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760454AbXGPTb1 (ORCPT ); Mon, 16 Jul 2007 15:31:27 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755719AbXGPTbU (ORCPT ); Mon, 16 Jul 2007 15:31:20 -0400 Received: from e5.ny.us.ibm.com ([32.97.182.145]:44991 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754443AbXGPTbT (ORCPT ); Mon, 16 Jul 2007 15:31:19 -0400 Subject: Re: [PATCH] Simplify /proc//exe symlink code From: Matt Helsley To: Al Viro Cc: Andrew Morton , Chris Wright , linux-mm@kvack.org, LKML , Christoph Hellwig , "Hallyn, Serge" , Peter Zijlstra In-Reply-To: <20070713020710.GA21668@ftp.linux.org.uk> References: <1184292012.13479.14.camel@localhost.localdomain> <20070713020710.GA21668@ftp.linux.org.uk> Content-Type: text/plain Organization: IBM Linux Technology Center Date: Mon, 16 Jul 2007 12:31:14 -0700 Message-Id: <1184614274.4877.49.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.6.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2007-07-13 at 03:07 +0100, Al Viro wrote: > On Thu, Jul 12, 2007 at 07:00:12PM -0700, Matt Helsley wrote: > > This patch avoids holding the mmap semaphore while walking VMAs in response to > > programs which read or follow the /proc//exe symlink. This also allows > > us to merge mmu and nommu proc_exe_link() functions. The costs are holding the > > task lock, a separate reference to the executable file stored in the task > > struct, and increased code in fork, exec, and exit paths. > > I don't think it's a food idea. Consider a program that deliberately > creates an executable anon memory, copies the binary there, jumps there > and unmaps the original. In the current tree you'll get nothing > pinning the binary; with your patch it will remained busy. Yes, it will prevent the filesystem with the executable file from being unmounted. Do you have an example where the original filesystem urgently needs to be unmounted while this unusual executable is running? Or is umount -l sufficient here? > It's not a common situation, of course, but there are legitimate uses > for such technics... Yes, I'm aware of at least one example where this technique has legitimate uses: libhugetlbfs. I'm interested in testing others you may be able to recommend as well. Furthermore, in your example the VMA walk would make /proc/self/exe a symlink to the file that backs the next executable VMA: libc, libdl, etc. That seems rather odd to me. In contrast, with my patch /proc/self/exe would always be a symlink to the original executable. Cheers, -Matt Helsley From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from d01relay02.pok.ibm.com (d01relay02.pok.ibm.com [9.56.227.234]) by e2.ny.us.ibm.com (8.13.8/8.13.8) with ESMTP id l6GJVHQh001905 for ; Mon, 16 Jul 2007 15:31:17 -0400 Received: from d01av01.pok.ibm.com (d01av01.pok.ibm.com [9.56.224.215]) by d01relay02.pok.ibm.com (8.13.8/8.13.8/NCO v8.4) with ESMTP id l6GJVHBd407474 for ; Mon, 16 Jul 2007 15:31:17 -0400 Received: from d01av01.pok.ibm.com (loopback [127.0.0.1]) by d01av01.pok.ibm.com (8.12.11.20060308/8.13.3) with ESMTP id l6GJVGGn029058 for ; Mon, 16 Jul 2007 15:31:17 -0400 Subject: Re: [PATCH] Simplify /proc//exe symlink code From: Matt Helsley In-Reply-To: <20070713020710.GA21668@ftp.linux.org.uk> References: <1184292012.13479.14.camel@localhost.localdomain> <20070713020710.GA21668@ftp.linux.org.uk> Content-Type: text/plain Date: Mon, 16 Jul 2007 12:31:14 -0700 Message-Id: <1184614274.4877.49.camel@localhost.localdomain> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Al Viro Cc: Andrew Morton , Chris Wright , linux-mm@kvack.org, LKML , Christoph Hellwig , "Hallyn, Serge" , Peter Zijlstra List-ID: On Fri, 2007-07-13 at 03:07 +0100, Al Viro wrote: > On Thu, Jul 12, 2007 at 07:00:12PM -0700, Matt Helsley wrote: > > This patch avoids holding the mmap semaphore while walking VMAs in response to > > programs which read or follow the /proc//exe symlink. This also allows > > us to merge mmu and nommu proc_exe_link() functions. The costs are holding the > > task lock, a separate reference to the executable file stored in the task > > struct, and increased code in fork, exec, and exit paths. > > I don't think it's a food idea. Consider a program that deliberately > creates an executable anon memory, copies the binary there, jumps there > and unmaps the original. In the current tree you'll get nothing > pinning the binary; with your patch it will remained busy. Yes, it will prevent the filesystem with the executable file from being unmounted. Do you have an example where the original filesystem urgently needs to be unmounted while this unusual executable is running? Or is umount -l sufficient here? > It's not a common situation, of course, but there are legitimate uses > for such technics... Yes, I'm aware of at least one example where this technique has legitimate uses: libhugetlbfs. I'm interested in testing others you may be able to recommend as well. Furthermore, in your example the VMA walk would make /proc/self/exe a symlink to the file that backs the next executable VMA: libc, libdl, etc. That seems rather odd to me. In contrast, with my patch /proc/self/exe would always be a symlink to the original executable. Cheers, -Matt Helsley -- 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