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Date:   Tue, 12 Nov 2019 11:16:58 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Chris Down <chris@chrisdown.name>, Qian Cai <cai@lca.pw>,
        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>
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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 <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 | 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