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Tue, 12 Nov 2019 08:16:59 -0800 (PST) Date: Tue, 12 Nov 2019 11:16:58 -0500 From: Johannes Weiner To: Michal Hocko Cc: Chris Down , Qian Cai , akpm@linux-foundation.org, guro@fb.com, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH -next] mm/vmscan: fix an undefined behavior for zone id Message-ID: <20191112161658.GF168812@cmpxchg.org> References: <20191108204407.1435-1-cai@lca.pw> <64E60F6F-7582-427B-8DD5-EF97B1656F5A@lca.pw> <20191111130516.GA891635@chrisdown.name> <20191111131427.GB891635@chrisdown.name> <20191111132812.GK1396@dhcp22.suse.cz> <20191112145942.GA168812@cmpxchg.org> <20191112152750.GA512@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191112152750.GA512@dhcp22.suse.cz> User-Agent: Mutt/1.12.2 (2019-09-21) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Nov 12, 2019 at 04:27:50PM +0100, Michal Hocko wrote: > On Tue 12-11-19 06:59:42, Johannes Weiner wrote: > > Qian, thanks for the report and the fix. > > > > On Mon, Nov 11, 2019 at 02:28:12PM +0100, Michal Hocko wrote: > > > On Mon 11-11-19 13:14:27, Chris Down wrote: > > > > Chris Down writes: > > > > > Ah, I just saw this in my local checkout and thought it was from my > > > > > changes, until I saw it's also on clean mmots checkout. Thanks for the > > > > > fixup! > > > > > > > > Also, does this mean we should change callers that may pass through > > > > zone_idx=MAX_NR_ZONES to become MAX_NR_ZONES-1 in a separate commit, then > > > > remove this interim fixup? I'm worried otherwise we might paper over real > > > > issues in future. > > > > > > Yes, removing this special casing is reasonable. I am not sure > > > MAX_NR_ZONES - 1 is a better choice though. It is error prone and > > > zone_idx is the highest zone we should consider and MAX_NR_ZONES - 1 > > > be ZONE_DEVICE if it is configured. But ZONE_DEVICE is really standing > > > outside of MM reclaim code AFAIK. It would be probably better to have > > > MAX_LRU_ZONE (equal to MOVABLE) and use it instead. > > > > We already use MAX_NR_ZONES - 1 everywhere else in vmscan.c to mean > > "no zone restrictions" - get_scan_count() is the odd one out: > > > > - mem_cgroup_shrink_node() > > - try_to_free_mem_cgroup_pages() > > - balance_pgdat() > > - kswapd() > > - shrink_all_memory() > > > > It's a little odd that it points to ZONE_DEVICE, but it's MUCH less > > subtle than handling both inclusive and exclusive range delimiters. > > > > So I think the better fix would be this: > > lruvec_lru_size is explicitly documented to use MAX_NR_ZONES for all > LRUs and git grep says there are more instances outside of > get_scan_count. So all of them have to be fixed. Which ones? [hannes@computer linux]$ git grep lruvec_lru_size include/linux/mmzone.h:extern unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone_idx); mm/vmscan.c: * lruvec_lru_size - Returns the number of pages on the given LRU list. mm/vmscan.c:unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone_idx) mm/vmscan.c: anon = lruvec_lru_size(lruvec, LRU_ACTIVE_ANON, MAX_NR_ZONES - 1) + mm/vmscan.c: lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, MAX_NR_ZONES - 1); mm/vmscan.c: file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES - 1) + mm/vmscan.c: lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, MAX_NR_ZONES - 1); mm/vmscan.c: lruvec_size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx); [hannes@computer linux]$ The only other user already passes sc->reclaim_idx, which always points to a valid zone, and is initialized to MAX_NR_ZONES - 1 in many places. > I still think that MAX_NR_ZONES - 1 is a very error prone and subtle > construct IMHO and an alias would be better readable. I wouldn't mind a follow-up patch that changes this pattern comprehensively. As it stands, get_scan_count() is the odd one out. The documentation bit is a good point, though. We should fix that. Updated patch: --- >From b1b6ce306010554aba6ebd7aac0abffc1576d71a Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Mon, 11 Nov 2019 13:46:25 -0800 Subject: [PATCH] mm: vmscan: simplify lruvec_lru_size() fix get_scan_count() passes MAX_NR_ZONES for the reclaim index, which is beyond the range of valid zone indexes, but used to be handled before the patch. Every other callsite in vmscan.c passes MAX_NR_ZONES - 1 to express "all zones, please", so do the same here. Reported-by: Qian Cai Reported-by: Chris Down Signed-off-by: Johannes Weiner --- mm/vmscan.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index df859b1d583c..5eb96a63ad1e 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -323,7 +323,7 @@ unsigned long zone_reclaimable_pages(struct zone *zone) * lruvec_lru_size - Returns the number of pages on the given LRU list. * @lruvec: lru vector * @lru: lru to use - * @zone_idx: zones to consider (use MAX_NR_ZONES for the whole LRU list) + * @zone_idx: index of the highest zone to include (use MAX_NR_ZONES - 1 for all) */ unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone_idx) { @@ -2322,10 +2322,10 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc, * anon in [0], file in [1] */ - anon = lruvec_lru_size(lruvec, LRU_ACTIVE_ANON, MAX_NR_ZONES) + - lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, MAX_NR_ZONES); - file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES) + - lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, MAX_NR_ZONES); + anon = lruvec_lru_size(lruvec, LRU_ACTIVE_ANON, MAX_NR_ZONES - 1) + + lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, MAX_NR_ZONES - 1); + file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES - 1) + + lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, MAX_NR_ZONES - 1); spin_lock_irq(&pgdat->lru_lock); if (unlikely(reclaim_stat->recent_scanned[0] > anon / 4)) { -- 2.24.0