From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935225AbcIWIrs (ORCPT ); Fri, 23 Sep 2016 04:47:48 -0400 Received: from outbound-smtp11.blacknight.com ([46.22.139.16]:34261 "EHLO outbound-smtp11.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754313AbcIWIrm (ORCPT ); Fri, 23 Sep 2016 04:47:42 -0400 Date: Fri, 23 Sep 2016 09:47:29 +0100 From: Mel Gorman To: Hugh Dickins Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm: delete unnecessary and unsafe init_tlb_ubc() Message-ID: <20160923084729.GA2838@techsingularity.net> References: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 22, 2016 at 10:41:50AM -0700, Hugh Dickins wrote: > init_tlb_ubc() looked unnecessary to me: tlb_ubc is statically initialized > with zeroes in the init_task, and copied from parent to child while it is > quiescent in arch_dup_task_struct(); so I went to delete it. > > But inserted temporary debug WARN_ONs in place of init_tlb_ubc() to check > that it was always empty at that point, and found them firing: because > memcg reclaim can recurse into global reclaim (when allocating biosets > for swapout in my case), and arrive back at the init_tlb_ubc() in > shrink_node_memcg(). > > Resetting tlb_ubc.flush_required at that point is wrong: if the upper > level needs a deferred TLB flush, but the lower level turns out not to, > we miss a TLB flush. But fortunately, that's the only part of the > protocol that does not nest: with the initialization removed, cpumask > collects bits from upper and lower levels, and flushes TLB when needed. > > Fixes: 72b252aed506 ("mm: send one IPI per CPU to TLB flush all entries after unmapping pages") > Signed-off-by: Hugh Dickins > Cc: stable@vger.kernel.org # 4.3+ Acked-by: Mel Gorman -- Mel Gorman SUSE Labs