linux-kernel - Re: [PATCH v2] mm/hugetlb: add mempolicy check in the reservation routine

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Date:   Fri, 24 Jul 2020 10:41:14 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>, akpm@...ux-foundation.org,
        mhocko@...nel.org
Cc:     rientjes@...gle.com, mgorman@...e.de, walken@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jianchao Guo <guojianchao@...edance.com>
Subject: Re: [PATCH v2] mm/hugetlb: add mempolicy check in the reservation
 routine

On 7/24/20 3:03 AM, 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
> 
> With this patch applied, the mmap will fail in the step 3) and throw
> "mmap: Cannot allocate memory".
> 
> Reported-by: Jianchao Guo <guojianchao@...edance.com>
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

Michal has already provides some good suggestions.

> @@ -3653,7 +3666,7 @@ static int hugetlb_acct_memory(struct hstate *h, long delta)
>  		if (gather_surplus_pages(h, delta) < 0)
>  			goto out;
>  
> -		if (delta > cpuset_mems_nr(h->free_huge_pages_node)) {
> +		if (delta > allowed_mems_nr(h)) {
>  			return_unused_surplus_pages(h, delta);
>  			goto out;
>  		}

There is a big comment before this code in hugetlb_acct_memory.  The comment
only talks about cpusets.  We should probably update that to include mempolicy
as well.  It could be as simple as s/cpuset/cpuset or mempolicy/.
-- 
Mike Kravetz