From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754154AbdGNNCu (ORCPT ); Fri, 14 Jul 2017 09:02:50 -0400 Received: from mx2.suse.de ([195.135.220.15]:53346 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753634AbdGNNCt (ORCPT ); Fri, 14 Jul 2017 09:02:49 -0400 Date: Fri, 14 Jul 2017 15:02:42 +0200 From: Michal Hocko To: Mel Gorman Cc: linux-mm@kvack.org, Andrew Morton , Johannes Weiner , Vlastimil Babka , LKML Subject: Re: [PATCH 6/9] mm, page_alloc: simplify zonelist initialization Message-ID: <20170714130242.GQ2618@dhcp22.suse.cz> References: <20170714080006.7250-1-mhocko@kernel.org> <20170714080006.7250-7-mhocko@kernel.org> <20170714124645.i3duhuie6cczlybr@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170714124645.i3duhuie6cczlybr@suse.de> 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 Fri 14-07-17 13:46:46, Mel Gorman wrote: > On Fri, Jul 14, 2017 at 10:00:03AM +0200, Michal Hocko wrote: > > From: Michal Hocko > > > > build_zonelists gradually builds zonelists from the nearest to the most > > distant node. As we do not know how many populated zones we will have in > > each node we rely on the _zoneref to terminate initialized part of the > > zonelist by a NULL zone. While this is functionally correct it is quite > > suboptimal because we cannot allow updaters to race with zonelists > > users because they could see an empty zonelist and fail the allocation > > or hit the OOM killer in the worst case. > > > > We can do much better, though. We can store the node ordering into an > > already existing node_order array and then give this array to > > build_zonelists_in_node_order and do the whole initialization at once. > > zonelists consumers still might see halfway initialized state but that > > should be much more tolerateable because the list will not be empty and > > they would either see some zone twice or skip over some zone(s) in the > > worst case which shouldn't lead to immediate failures. > > > > This patch alone doesn't introduce any functional change yet, though, it > > is merely a preparatory work for later changes. > > > > Signed-off-by: Michal Hocko > > --- > > mm/page_alloc.c | 42 ++++++++++++++++++------------------------ > > 1 file changed, 18 insertions(+), 24 deletions(-) > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > index 00e117922b3f..78bd62418380 100644 > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -4913,17 +4913,20 @@ static int find_next_best_node(int node, nodemask_t *used_node_mask) > > * This results in maximum locality--normal zone overflows into local > > * DMA zone, if any--but risks exhausting DMA zone. > > */ > > -static void build_zonelists_in_node_order(pg_data_t *pgdat, int node) > > +static void build_zonelists_in_node_order(pg_data_t *pgdat, int *node_order) > > { > > - int j; > > struct zonelist *zonelist; > > + int i, zoneref_idx = 0; > > > > zonelist = &pgdat->node_zonelists[ZONELIST_FALLBACK]; > > - for (j = 0; zonelist->_zonerefs[j].zone != NULL; j++) > > - ; > > - j = build_zonelists_node(NODE_DATA(node), zonelist, j); > > - zonelist->_zonerefs[j].zone = NULL; > > - zonelist->_zonerefs[j].zone_idx = 0; > > + > > + for (i = 0; i < MAX_NUMNODES; i++) { > > + pg_data_t *node = NODE_DATA(node_order[i]); > > + > > + zoneref_idx = build_zonelists_node(node, zonelist, zoneref_idx); > > + } > > The naming here is weird to say the least and makes this a lot more > confusing than it needs to be. Primarily, it's because the zoneref_idx > parameter gets renamed to nr_zones in build_zonelists_node where it's > nothing to do with the number of zones at all. you are right. I just wanted to get rid of `j' and didn't realize nr_zones would fit much better. > It also iterates for longer than it needs to. MAX_NUMNODES can be a > large value of mostly empty nodes but it happily goes through them > anyway. Pass zoneref_idx in as a pointer that is updated by the function > and use the return value to break the loop when an empty node is > encountered? > > > + zonelist->_zonerefs[zoneref_idx].zone = NULL; > > + zonelist->_zonerefs[zoneref_idx].zone_idx = 0; > > } > > > > It *might* be safer given the next patch to zero out the remainder of > the _zonerefs to that there is no combination of node add/remove that has > an iterator working with a semi-valid _zoneref which is beyond the last > correct value. It *should* be safe as the very last entry will always > be null but if you don't zero it out, it is possible for iterators to be > working beyond the "end" of the zonelist for a short window. yes that is true but there will always be terminating NULL zone and I found that acceptable. It is basically the same thing as accessing an empty zone or a zone twice. Or do you think this is absolutely necessary to handle? > Otherwise think it's ok including my stupid comment about node_order > stack usage. What do you think about this on top? --- diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 49bade7ff049..3b98524c04ec 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -4913,20 +4913,21 @@ static int find_next_best_node(int node, nodemask_t *used_node_mask) * This results in maximum locality--normal zone overflows into local * DMA zone, if any--but risks exhausting DMA zone. */ -static void build_zonelists_in_node_order(pg_data_t *pgdat, int *node_order) +static void build_zonelists_in_node_order(pg_data_t *pgdat, int *node_order, + unsigned nr_nodes) { struct zonelist *zonelist; - int i, zoneref_idx = 0; + int i, nr_zones = 0; zonelist = &pgdat->node_zonelists[ZONELIST_FALLBACK]; - for (i = 0; i < MAX_NUMNODES; i++) { + for (i = 0; i < nr_nodes; i++) { pg_data_t *node = NODE_DATA(node_order[i]); - zoneref_idx = build_zonelists_node(node, zonelist, zoneref_idx); + nr_zones = build_zonelists_node(node, zonelist, nr_zones); } - zonelist->_zonerefs[zoneref_idx].zone = NULL; - zonelist->_zonerefs[zoneref_idx].zone_idx = 0; + zonelist->_zonerefs[nr_zones].zone = NULL; + zonelist->_zonerefs[nr_zones].zone_idx = 0; } /* @@ -4935,12 +4936,12 @@ static void build_zonelists_in_node_order(pg_data_t *pgdat, int *node_order) static void build_thisnode_zonelists(pg_data_t *pgdat) { struct zonelist *zonelist; - int zoneref_idx = 0; + int nr_zones = 0; zonelist = &pgdat->node_zonelists[ZONELIST_NOFALLBACK]; - zoneref_idx = build_zonelists_node(pgdat, zonelist, zoneref_idx); - zonelist->_zonerefs[zoneref_idx].zone = NULL; - zonelist->_zonerefs[zoneref_idx].zone_idx = 0; + nr_zones = build_zonelists_node(pgdat, zonelist, nr_zones); + zonelist->_zonerefs[nr_zones].zone = NULL; + zonelist->_zonerefs[nr_zones].zone_idx = 0; } /* @@ -4979,7 +4980,7 @@ static void build_zonelists(pg_data_t *pgdat) load--; } - build_zonelists_in_node_order(pgdat, node_order); + build_zonelists_in_node_order(pgdat, node_order, i); build_thisnode_zonelists(pgdat); } -- Michal Hocko SUSE Labs