linux-kernel - Re: [PATCH] mm: fix setting the high and low watermarks

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Message-ID: <c997fb4d-c064-4faf-f40b-1fda064681b1@suse.cz>
Date:   Mon, 24 Jun 2019 07:46:53 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Bharath Vedartham <linux.bhar@...il.com>
Cc:     Alan Jenkins <alan.christopher.jenkins@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm: fix setting the high and low watermarks

On 6/21/19 4:07 PM, Bharath Vedartham wrote:
> Do you think this could cause a race condition between
> __setup_per_zone_wmarks and pgdat_watermark_boosted which checks whether
> the watermark_boost of each zone is non-zero? pgdat_watermark_boosted is
> not called with a zone lock.
> Here is a probable case scenario:
> watermarks are boosted in steal_suitable_fallback(which happens under a
> zone lock). After that kswapd is woken up by
> wakeup_kswapd(zone,0,0,zone_idx(zone)) in rmqueue without holding a
> zone lock. Lets say someone modified min_kfree_bytes, this would lead to
> all the zone->watermark_boost being set to 0. This may cause
> pgdat_watermark_boosted to return false, which would not wakeup kswapd
> as intended by boosting the watermark. This behaviour is similar to waking up kswapd for a
> balanced node.

Not waking up kswapd shouldn't cause a significant trouble.

> Also if kswapd was woken up successfully because of watermarks being
> boosted. In balance_pgdat, we use nr_boost_reclaim to count number of
> pages to reclaim because of boosting. nr_boost_reclaim is calculated as:
> nr_boost_reclaim = 0;
> for (i = 0; i <= classzone_idx; i++) {
> 	zone = pgdat->node_zones + i;
> 	if (!managed_zone(zone))
> 		continue;
> 
> 	nr_boost_reclaim += zone->watermark_boost;
> 	zone_boosts[i] = zone->watermark_boost;
> }
> boosted = nr_boost_reclaim;
> 
> This is not under a zone_lock. This could lead to nr_boost_reclaim to
> be 0 if min_kfree_bytes is set to 0. Which would wake up kcompactd
> without reclaiming memory.

Setting min_kfree_bytes to 0 is asking for problems regardless of this
check. Much more trouble than waking up kcompactd spuriously, which is
just a few wasted cpu cycles.

> kcompactd compaction might be spurious if the if the memory reclaim step is not happening?
> 
> Any thoughts?

Unless the races cause either some data corruption, or e.g. spurious
allocation failures, I don't think they are worth adding new spinlock
sections.

Thanks,
Vlastimil

>>  		spin_unlock_irqrestore(&zone->lock, flags);
>>
>