From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934219AbaDIRxe (ORCPT ); Wed, 9 Apr 2014 13:53:34 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:58007 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933323AbaDIRxa (ORCPT ); Wed, 9 Apr 2014 13:53:30 -0400 Date: Wed, 9 Apr 2014 18:53:23 +0100 From: Al Viro To: "Eric W. Biederman" Cc: Linus Torvalds , "Serge E. Hallyn" , Linux-Fsdevel , Kernel Mailing List , Andy Lutomirski , Rob Landley , Miklos Szeredi , Christoph Hellwig , Karel Zak , "J. Bruce Fields" , Fengguang Wu Subject: Re: [GIT PULL] Detaching mounts on unlink for 3.15-rc1 Message-ID: <20140409175322.GZ18016@ZenIV.linux.org.uk> References: <8761v7h2pt.fsf@tw-ebiederman.twitter.com> <87li281wx6.fsf_-_@xmission.com> <87ob28kqks.fsf_-_@xmission.com> <874n3n7czm.fsf_-_@xmission.com> <87wqezl5df.fsf_-_@x220.int.ebiederm.org> <20140409023027.GX18016@ZenIV.linux.org.uk> <20140409023947.GY18016@ZenIV.linux.org.uk> <87sipmbe8x.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87sipmbe8x.fsf@x220.int.ebiederm.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 09, 2014 at 10:32:14AM -0700, Eric W. Biederman wrote: > For resolving a deeply nested symlink that hits the limit of 8 nested > symlinks, I find 4688 bytes left on the stack. Which means we use > roughly 3504 bytes of stack when stating a deeply nested symlink. > > For umount I had a little trouble measuring as typically the work done > by umount was not the largest stack consumer, but I found for a small > ext4 filesystem after the umount operation was complete there were > 5152 bytes left on the stack, or umount used roughly 3040 bytes. A bit less - we have a non-empty stack footprint from sys_umount() itself. > 3504 + 3040 = 6544 bytes of stack used or 1684 bytes of stack left > unused. Which certainly isn't a lot of margin but it is not overflowing > the kernel stack either. > > Is there a case that see where umount uses a lot more kernel stack? Is > your concern an architecture other than x86_64 with different > limitations? For starters, put that ext4 on top of dm-raid or dm-multipath. That alone will very likely push you over the top. Keep in mind, BTW, that you do not have full 8K to play with - there's struct thread_info that should not be stepped upon. Not particulary large (IIRC, restart_block is the largest piece in amd64 one), but it eats about 100 bytes. I'd probably use renameat(2) in testing - i.e. trigger the shite when resolving a deeply nested symlink in renameat() arguments. That brings extra struct nameidata into the game, i.e. extra 152 bytes chewed off the stack.