From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751208AbdBUWze (ORCPT ); Tue, 21 Feb 2017 17:55:34 -0500 Received: from ipmail04.adl6.internode.on.net ([150.101.137.141]:13411 "EHLO ipmail04.adl6.internode.on.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753432AbdBUWyR (ORCPT ); Tue, 21 Feb 2017 17:54:17 -0500 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: A2A/WgBuxKxYEFkELHleHAEBBAEBCgEBg1GBaocGh0+PdgEBAQEBAQaBIoxOhleEHIR3gSUEAgKCcFgBAgEBAQEBAgYBAQEBAQEBAThEhHEBBScTHCMQCAMYCSUPBSUDBxoTiWEMsRM6iz4BAQEBBgEBAQEkIIVnhTSICIIxAQScC5ISkRxIkl1WgQEgFAgXFYU+gVwsNYo6AQEB Date: Wed, 22 Feb 2017 09:54:02 +1100 From: Dave Chinner To: Peter Zijlstra Cc: Elena Reshetova , linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, gregkh@linuxfoundation.org, darrick.wong@oracle.com, Hans Liljestrand , Kees Cook , David Windsor Subject: Re: [PATCH 2/7] fs, xfs: convert xfs_buf.b_hold and xfs_buf.b_lru_ref from atomic_t to refcount_t Message-ID: <20170221225402.GA23007@dastard> References: <1487692147-17066-1-git-send-email-elena.reshetova@intel.com> <1487692147-17066-3-git-send-email-elena.reshetova@intel.com> <20170221160408.GJ6515@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170221160408.GJ6515@twins.programming.kicks-ass.net> 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 Tue, Feb 21, 2017 at 05:04:08PM +0100, Peter Zijlstra wrote: > On Tue, Feb 21, 2017 at 05:49:02PM +0200, Elena Reshetova wrote: > > @@ -1684,10 +1684,11 @@ xfs_buftarg_isolate( > > * zero. If the value is already zero, we need to reclaim the > > * buffer, otherwise it gets another trip through the LRU. > > */ > > - if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) { > > + if (!refcount_read(&bp->b_lru_ref)) { > > spin_unlock(&bp->b_lock); > > return LRU_ROTATE; > > } > > + refcount_dec_and_test(&bp->b_lru_ref); > > > > bp->b_state |= XFS_BSTATE_DISPOSE; > > list_lru_isolate_move(lru, item, dispose); > > This should never have passed testing.. refcount_dec_and_test() has a > __must_check. > > Furthermore the above seems to suggest thingies can live with a 0 > refcount, so a straight conversion cannot work. Yes, 0 is a valid value - the buffer lru reference is *not an object lifecycle reference count*. A value of zero means reclaim needs to take action if it sees that value - it does not mean that the object is not referenced by anyone (that's b_hold). i.e. b_lru_ref is an "active reference weighting" used to provide a heirarchical reclaim bias toward less important metadata objects, and has no bearing on the actual active users of the object. Cheers, Dave. -- Dave Chinner david@fromorbit.com