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Subject: Re: [PATCH v4] mm/hugetlb: add mempolicy check in the reservation
 routine
To:     Baoquan He <bhe@redhat.com>, Muchun Song <songmuchun@bytedance.com>
Cc:     akpm@linux-foundation.org, mhocko@kernel.org, rientjes@google.com,
        mgorman@suse.de, walken@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Jianchao Guo <guojianchao@bytedance.com>
References: <20200728034938.14993-1-songmuchun@bytedance.com>
 <20200728132453.GB14854@MiWiFi-R3L-srv>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <1b507031-d475-b495-bb4a-2cd9e665d02f@oracle.com>
Date:   Tue, 28 Jul 2020 09:46:02 -0700
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On 7/28/20 6:24 AM, Baoquan He wrote:
> Hi Muchun,
> 
> On 07/28/20 at 11:49am, Muchun Song wrote:
>> In the reservation routine, we only check whether the cpuset meets
>> the memory allocation requirements. But we ignore the mempolicy of
>> MPOL_BIND case. If someone mmap hugetlb succeeds, but the subsequent
>> memory allocation may fail due to mempolicy restrictions and receives
>> the SIGBUS signal. This can be reproduced by the follow steps.
>>
>>  1) Compile the test case.
>>     cd tools/testing/selftests/vm/
>>     gcc map_hugetlb.c -o map_hugetlb
>>
>>  2) Pre-allocate huge pages. Suppose there are 2 numa nodes in the
>>     system. Each node will pre-allocate one huge page.
>>     echo 2 > /proc/sys/vm/nr_hugepages
>>
>>  3) Run test case(mmap 4MB). We receive the SIGBUS signal.
>>     numactl --membind=0 ./map_hugetlb 4
> 
> I think supporting the  mempolicy of MPOL_BIND case is a good idea.
> I am wondering what about the other mempolicy cases, e.g MPOL_INTERLEAVE,
> MPOL_PREFERRED. Asking these because we already have similar handling in
> sysfs, proc nr_hugepages_mempolicy writting. Please see
> __nr_hugepages_store_common() for detail.

There is a high level difference in the function of this code and the code
called by the sysfs and proc interfaces.  This patch is dealing with reserving
huge pages in the pool for later use.  The sysfs and proc interfaces are
allocating huge pages to be added to the pool.

Using mempolicy to decide how to allocate huge pages is pretty straight
forward.  Using mempolicy to reserve pages is almost impossible to get
correct.  The comment at the beginning of hugetlb_acct_memory() and modified
by this patch summarizes the issues.

IMO, at this time it makes little sense to perform checks for more than
MPOL_BIND at reservation time.  If we ever take on the monumental task of
supporting mempolicy directed per-node reservations throughout the life of
a process, support for other policies will need to be taken into account.

-- 
Mike Kravetz