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Date:   Mon, 7 Jun 2021 10:10:08 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     kernel test robot <oliver.sang@intel.com>
Cc:     Feng Tang <feng.tang@intel.com>, 0day robot <lkp@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        ltp@lists.linux.it, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Ben Widawsky <ben.widawsky@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andi Kleen <ak@linux.intel.com>,
        Dan Williams <dan.j.williams@intel.com>, ying.huang@intel.com
Subject: Re: [mm/mempolicy]  7463fff037: ltp.mbind01.fail
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 <20210607074815.GC16270@xsang-OptiPlex-9020>
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On Mon 07-06-21 15:48:15, kernel test robot wrote:
> mbind01.c:169: TINFO: case MPOL_PREFERRED (no target)
> mbind01.c:188: TFAIL: Wrong policy: 1, expected: 4

AFAIU this points to
static void test_none(unsigned int i, char *p)
{
        struct test_case *tc = &tcase[i];

        TEST(mbind(p, MEM_LENGTH, tc->policy, NULL, 0, tc->flags));
}

So it calls MPOL_PREFERRED with NULL parameter and the test has failed
because the kernel returns MPOL_LOCAL instead of MPOL_PREFERRED. Strictly
speaking this is breaking user interface but I am wondering whether this
really matter or is completely unexpected.  The manual page explicitly
talks about this case
"
	If the nodemask and maxnode arguments specify the empty set, then
	the memory is allocated on the node of the CPU that triggered the
	allocation.
"

I would be inclined to keep this inconsistency and see whether anybody
actually complains and have a relevant use for this behavior. The
cleanup which makes the code easier to maintain and less error prone
doesn't really deserve to get ditched just because of this IMHO.
-- 
Michal Hocko
SUSE Labs