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Subject: Re: [PATCH -next v2] mm: mark an intentional data race in
 page_zonenum
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <20200206202005.GY8731@bombadil.infradead.org>
Date:   Thu, 6 Feb 2020 15:25:41 -0500
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Marco Elver <elver@google.com>, jhubbard@nvidia.com,
        ira.weiny@intel.com, dan.j.williams@intel.com, jack@suse.cz,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
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References: <20200206183000.913-1-cai@lca.pw>
 <20200206202005.GY8731@bombadil.infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Feb 6, 2020, at 3:20 PM, Matthew Wilcox <willy@infradead.org> =
wrote:
>=20
> On Thu, Feb 06, 2020 at 01:30:00PM -0500, Qian Cai wrote:
>> Both the read and write are done only with the non-exclusive mmap_sem
>> held. Since the read only check for a specific bit range (up to 3 =
bits)
>> in the flag but the write here never change those 3 bits, so load
>> tearing would be harmless here. Thus, just mark it as an intentional
>> data races using the data_race() macro which is designed for those
>> situations [1].
>=20
> This changelog makes me think you don't really understand the =
situation.
>=20
> A page never changes its zone number.  The zone number happens to be
> stored in the same word as other bits which are modified, but the zone
> number bits will never be modified by any other write.  So we can =
accept
> a reload of the zone bits after an intervening write and we don't need
> to use READ_ONCE().

Maybe your explanation is better, but I did try to explain the same =
thing.
I=E2=80=99ll let Andrew to decide if he would like to update the commit =
log a bit
with your wording.=