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

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Message-ID: <20191111132812.GK1396@dhcp22.suse.cz>
Date:   Mon, 11 Nov 2019 14:28:12 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Chris Down <chris@...isdown.name>
Cc:     Qian Cai <cai@....pw>, akpm@...ux-foundation.org,
        hannes@...xchg.org, guro@...com, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] mm/vmscan: fix an undefined behavior for zone id

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.

-- 
Michal Hocko
SUSE Labs