From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Chinner Subject: Re: DNAME_INLINE_LEN versus CONFIG_GENERIC_LOCKBREAK Date: Fri, 4 Jul 2014 11:32:22 +1000 Message-ID: <20140704013222.GH9508@dastard> References: <8761je94wk.fsf@rasmusvillemoes.dk> <87fviite1u.fsf@tassilo.jf.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Rasmus Villemoes , Nick Piggin , Al Viro , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org To: Andi Kleen Return-path: Content-Disposition: inline In-Reply-To: <87fviite1u.fsf@tassilo.jf.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Thu, Jul 03, 2014 at 12:53:01PM -0700, Andi Kleen wrote: > Rasmus Villemoes writes: > > > In dcache.h, DNAME_INLINE_LEN is carefully chosen so that sizeof(struct > > dentry) is a (specific) multiple of 64 bytes. Obviously this breaks when > > certain debug options are chosen (DEBUG_LOCK_ALLOC and DEBUG_SPINLOCK), > > but also, AFAICT, on architectures with CONFIG_GENERIC_LOCKBREAK. > > > > I'm not sure it matters, but if it does, I'd suggest putting a > > BUILD_BUG_ON somewhere, protected by suitable #ifdefs, so that the code > > documents the assumptions that went into the particular choice of > > DNAME_INLINE_LEN (this would also help catch changes to some of the > > structures embedded in struct dentry which would violate those > > assumptions). > > The right fix would be to pad it correctly for these other variants > too. IF you've turned on debugging options, then you've already lost more performance that careful packing of the dentry slab cache gains you. There's no point in carefully tuning DNAME_INLINE_LEN for debug options - it's just code that will break and annoy people as debug implementations change. Cheers, Dave. -- Dave Chinner david@fromorbit.com