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[66.187.233.206]) by smtp.gmail.com with ESMTPSA id 189sm9518840qki.10.2019.11.12.08.24.52 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 12 Nov 2019 08:24:53 -0800 (PST) Message-ID: <1573575891.5937.118.camel@lca.pw> Subject: Re: [PATCH -next] mm/vmscan: fix an undefined behavior for zone id From: Qian Cai To: Johannes Weiner , Michal Hocko Cc: Chris Down , akpm@linux-foundation.org, guro@fb.com, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Date: Tue, 12 Nov 2019 11:24:51 -0500 In-Reply-To: <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> <20191112161658.GF168812@cmpxchg.org> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.22.6 (3.22.6-10.el7) Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable 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, 2019-11-12 at 11:16 -0500, Johannes Weiner wrote: > 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. > > >=20 > > > 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 f= rom my > > > > > > changes, until I saw it's also on clean mmots checkout. Thank= s for the > > > > > > fixup! > > > > >=20 > > > > > Also, does this mean we should change callers that may pass thr= ough > > > > > zone_idx=3DMAX_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. > > > >=20 > > > > 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 sta= nding > > > > outside of MM reclaim code AFAIK. It would be probably better to = have > > > > MAX_LRU_ZONE (equal to MOVABLE) and use it instead. > > >=20 > > > 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: > > >=20 > > > - mem_cgroup_shrink_node() > > > - try_to_free_mem_cgroup_pages() > > > - balance_pgdat() > > > - kswapd() > > > - shrink_all_memory() There is also=C2=A0inactive_list_is_low(), if (trace) trace_mm_vmscan_inactive_list_is_low(pgdat->node_id, sc->reclaim_idx, lruvec_lru_size(lruvec, inactive_lru, MAX_NR_ZONES), inactive, lruvec_lru_size(lruvec, active_lru, MAX_NR_ZONES), active, inactive_ratio, file); > > >=20 > > > 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. > > >=20 > > > So I think the better fix would be this: > >=20 > > 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. >=20 > Which ones? >=20 > [hannes@computer linux]$ git grep lruvec_lru_size > include/linux/mmzone.h:extern unsigned long lruvec_lru_size(struct lruv= ec *lruvec, enum lru_list lru, int zone_idx); > mm/vmscan.c: * lruvec_lru_size - Returns the number of pages on the gi= ven LRU list. > mm/vmscan.c:unsigned long lruvec_lru_size(struct lruvec *lruvec, enum l= ru_list lru, int zone_idx) > mm/vmscan.c: anon =3D 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 =3D 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 =3D lruvec_lru_size(lruvec, lru, sc= ->reclaim_idx); > [hannes@computer linux]$ >=20 > 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. >=20 > > I still think that MAX_NR_ZONES - 1 is a very error prone and subtle > > construct IMHO and an alias would be better readable. >=20 > I wouldn't mind a follow-up patch that changes this pattern > comprehensively. As it stands, get_scan_count() is the odd one out. >=20 > The documentation bit is a good point, though. We should fix > that. Updated patch: >=20 > --- >=20 > 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 >=20 > 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. >=20 > Reported-by: Qian Cai > Reported-by: Chris Down > Signed-off-by: Johannes Weiner > --- > mm/vmscan.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) >=20 > 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 *z= one) > * lruvec_lru_size - Returns the number of pages on the given LRU lis= t. > * @lruvec: lru vector > * @lru: lru to use > - * @zone_idx: zones to consider (use MAX_NR_ZONES for the whole LRU li= st) > + * @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 *lruve= c, struct scan_control *sc, > * anon in [0], file in [1] > */ > =20 > - anon =3D lruvec_lru_size(lruvec, LRU_ACTIVE_ANON, MAX_NR_ZONES) + > - lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, MAX_NR_ZONES); > - file =3D lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES) + > - lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, MAX_NR_ZONES); > + anon =3D lruvec_lru_size(lruvec, LRU_ACTIVE_ANON, MAX_NR_ZONES - 1) = + > + lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, MAX_NR_ZONES - 1); > + file =3D lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES - 1) = + > + lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, MAX_NR_ZONES - 1); > =20 > spin_lock_irq(&pgdat->lru_lock); > if (unlikely(reclaim_stat->recent_scanned[0] > anon / 4)) {