Re: [PATCH -next] mm/vmscan: fix an undefined behavior for zone id

[Johannes Weiner <hannes@cmpxchg.org>]

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:

---
From 1566a255eef7c2165d435125231ad1eeecac7959 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@cmpxchg.org>
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 <cai@lca.pw>
Reported-by: Chris Down <chris@chrisdown.name>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/vmscan.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index df859b1d583c..34ad8a0f3f27 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -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