Re: [bug report] hugetlb, mempolicy: fix the mbind hugetlb migration

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On 01/17/2018 09:18 PM, Michal Hocko wrote:
> On Wed 10-01-18 11:47:12, Michal Hocko wrote:
>> [CC Mike and Naoya]
> 
> ping
> 
>> From 7227218bd526cceb954a688727d78af0b5874e18 Mon Sep 17 00:00:00 2001
>> From: Michal Hocko <mhocko@suse.com>
>> Date: Wed, 10 Jan 2018 11:40:20 +0100
>> Subject: [PATCH] hugetlb, mbind: fall back to default policy if vma is NULL
>>
>> Dan Carpenter has noticed that mbind migration callback (new_page)
>> can get a NULL vma pointer and choke on it inside alloc_huge_page_vma
>> which relies on the VMA to get the hstate. We used to BUG_ON this
>> case but the BUG_+ON has been removed recently by "hugetlb, mempolicy:
>> fix the mbind hugetlb migration".
>>
>> The proper way to handle this is to get the hstate from the migrated
>> page and rely on huge_node (resp. get_vma_policy) do the right thing
>> with null VMA. We are currently falling back to the default mempolicy in
>> that case which is in line what THP path is doing here.

vma is used only for getting mempolicy in alloc_huge_page_vma(), so
falling back to default mempolicy looks better to me than BUG_ON.

Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Thanks,
Naoya Horiguchi
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