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Subject: Re: [PATCH] mm/vmscan: fix data races at kswapd_classzone_idx
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <20200226012952.GV24185@bombadil.infradead.org>
Date:   Tue, 25 Feb 2020 20:48:14 -0500
Cc:     Andrew Morton <akpm@linux-foundation.org>, elver@google.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
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 <20200226012952.GV24185@bombadil.infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
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> On Feb 25, 2020, at 8:29 PM, Matthew Wilcox <willy@infradead.org> =
wrote:
>=20
> On Tue, Feb 25, 2020 at 11:55:26AM -0500, Qian Cai wrote:
>> -	if (pgdat->kswapd_classzone_idx =3D=3D MAX_NR_ZONES)
>> -		pgdat->kswapd_classzone_idx =3D classzone_idx;
>> -	else
>> -		pgdat->kswapd_classzone_idx =3D =
max(pgdat->kswapd_classzone_idx,
>> -						  classzone_idx);
>> +	if (READ_ONCE(pgdat->kswapd_classzone_idx) =3D=3D MAX_NR_ZONES =
||
>> +	    READ_ONCE(pgdat->kswapd_classzone_idx) < classzone_idx)
>> +		WRITE_ONCE(pgdat->kswapd_classzone_idx, classzone_idx);
>> +
>> 	pgdat->kswapd_order =3D max(pgdat->kswapd_order, order);
>=20
> Doesn't this line have the exact same problem you're "solving" above?

Good catch. I missed that.

>=20
> Also, why would you do this crazy "f(READ_ONCE(x)) || g(READ_ONCE(x))?
> Surely you should be stashing READ_ONCE(x) into a local variable?

Not a bad idea.

>=20
> And there are a _lot_ of places which access kswapd_classzone_idx
> without a lock.  Are you sure this patch is sufficient?

I am not sure, but KCSAN did not complain about other places after =
running extended
period of testing, so I will look into once other places show up later.