linux-kernel - Re: [PATCH 02/10] mm/hugetlb: make sure to get NULL when list is empty

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Message-ID: <20200810230544.GA27669@L-31X9LVDL-1304.local>
Date:   Tue, 11 Aug 2020 07:05:44 +0800
From:   Wei Yang <richard.weiyang@...ux.alibaba.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Baoquan He <bhe@...hat.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/10] mm/hugetlb: make sure to get NULL when list is
 empty

On Mon, Aug 10, 2020 at 01:28:46PM -0700, Mike Kravetz wrote:
>On 8/7/20 7:28 AM, Wei Yang wrote:
>> On Fri, Aug 07, 2020 at 08:49:51PM +0800, Baoquan He wrote:
>>> On 08/07/20 at 05:12pm, Wei Yang wrote:
>>>> list_first_entry() may not return NULL even when the list is empty.
>>>>
>>>> Let's make sure the behavior by using list_first_entry_or_null(),
>>>> otherwise it would corrupt the list.
>>>>
>>>> Signed-off-by: Wei Yang <richard.weiyang@...ux.alibaba.com>
>>>> ---
>>>>  mm/hugetlb.c | 3 ++-
>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>> index 62ec74f6d03f..0a2f3851b828 100644
>>>> --- a/mm/hugetlb.c
>>>> +++ b/mm/hugetlb.c
>>>> @@ -237,7 +237,8 @@ get_file_region_entry_from_cache(struct resv_map *resv, long from, long to)
>>>>  	VM_BUG_ON(resv->region_cache_count <= 0);
>>>
>>>
>>> We have had above line, is it possible to be NULL from list_first_entry?
>>>
>>>>  
>>>>  	resv->region_cache_count--;
>>>> -	nrg = list_first_entry(&resv->region_cache, struct file_region, link);
>>>> +	nrg = list_first_entry_or_null(&resv->region_cache,
>>>> +			struct file_region, link);
>>>>  	VM_BUG_ON(!nrg);
>> 
>> Or we can remove this VM_BUG_ON()?
>> 
>
>I would prefer that we just remove the 'VM_BUG_ON(!nrg)'.  Code elsewhere
>is responsible for making sure there is ALWAYS an entry in the cache.  That
>is why the 'VM_BUG_ON(resv->region_cache_count <= 0)' is at the beginning
>of the routine.

Sure, will change to this.

>
>-- 
>Mike Kravetz

-- 
Wei Yang
Help you, Help me