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

[Michal Hocko <mhocko@kernel.org>]

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.

I still think that MAX_NR_ZONES - 1 is a very error prone and subtle
construct IMHO and an alias would be better readable.

Anyway I definitely do agree that we do not want to use both
(MAX_NR_ZONES and MAX_NR_ZONES - 1) because that is even more confusing.

> ---
> >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

-- 
Michal Hocko
SUSE Labs