linux-kernel - Re: [PATCH v3] mm/hugetlb: add mempolicy check in the reservation routine

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Message-ID: <833be473-a065-4402-f369-f946b6f4e312@oracle.com>
Date:   Mon, 27 Jul 2020 19:42:59 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <songmuchun@...edance.com>
Cc:     mhocko@...nel.org, 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 v3] mm/hugetlb: add mempolicy check in the reservation
 routine

On 7/27/20 5:19 PM, Andrew Morton wrote:
> On Sat, 25 Jul 2020 16:07:49 +0800 Muchun Song <songmuchun@...edance.com> 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".
> 
> This doesn't compile with CONFIG_NUMA=n - ther eis no implementation of
> get_task_policy().
> 
> I think it needs more than a simple build fix - can we please rework
> the patch so that its impact (mainly code size) on non-NUMA machines is
> minimized?

I'll let Muchun see if there is a more elegant fix.  However, a relatively
simple build fix such as:

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 8069ca47c18c..4bfbddfee0d3 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3455,12 +3455,14 @@ static unsigned int allowed_mems_nr(struct hstate *h)
 {
 	int node;
 	unsigned int nr = 0;
-	struct mempolicy *mpol = get_task_policy(current);
-	nodemask_t *mpol_allowed;
+	nodemask_t *mpol_allowed = NULL;
 	unsigned int *array = h->free_huge_pages_node;
+#ifdef CONFIG_NUMA
+	struct mempolicy *mpol = get_task_policy(current);
 	gfp_t gfp_mask = htlb_alloc_mask(h);

 	mpol_allowed = policy_nodemask(gfp_mask, mpol);
+#endif

 	for_each_node_mask(node, cpuset_current_mems_allowed) {
 		if (!mpol_allowed ||

Does not have much of an impact on code size.  Here are the non-numa
versions of the routine before Muchun's patch and after.

Dump of assembler code for function cpuset_mems_nr:
   0xffffffff8126a3a0 <+0>:	callq  0xffffffff81060f80 <__fentry__>
   0xffffffff8126a3a5 <+5>:	xor    %eax,%eax
   0xffffffff8126a3a7 <+7>:	mov    %gs:0x17bc0,%rdx
   0xffffffff8126a3b0 <+16>:	testb  $0x1,0x778(%rdx)
   0xffffffff8126a3b7 <+23>:	jne    0xffffffff8126a3ba <cpuset_mems_nr+26>
   0xffffffff8126a3b9 <+25>:	retq   
   0xffffffff8126a3ba <+26>:	mov    (%rdi),%eax
   0xffffffff8126a3bc <+28>:	retq   
End of assembler dump.

Dump of assembler code for function allowed_mems_nr:
   0xffffffff8126a3a0 <+0>:	callq  0xffffffff81060f80 <__fentry__>
   0xffffffff8126a3a5 <+5>:	xor    %eax,%eax
   0xffffffff8126a3a7 <+7>:	mov    %gs:0x17bc0,%rdx
   0xffffffff8126a3b0 <+16>:	testb  $0x1,0x778(%rdx)
   0xffffffff8126a3b7 <+23>:	jne    0xffffffff8126a3ba <allowed_mems_nr+26>
   0xffffffff8126a3b9 <+25>:	retq   
   0xffffffff8126a3ba <+26>:	mov    0x6c(%rdi),%eax
   0xffffffff8126a3bd <+29>:	retq   
End of assembler dump.

-- 
Mike Kravetz