Re: [PATCH v2] mm: fix the memory leak after collapsing the huge page fails

[Vlastimil Babka <vbabka@suse.cz>]

On 05/09/2017 03:54 PM, zhong jiang wrote:
> Hi, Vlastimil
> 
> I review the code again. it works well for NUMA. because
> khugepaged_prealloc_page will put_page when *hpage is true.
> 
> the memory leak will still exist in !NUMA. because it ingore
> the put_page. is it right? I miss something.

No, on !NUMA the preallocated and unused new_page is freed by put_page()
at the very end of khugepaged_do_scan().

> Thanks
> zhongjiang
> 
> On 2017/5/9 20:41, Vlastimil Babka wrote:
>> On 05/09/2017 02:20 PM, zhong jiang wrote:
>>> On 2017/5/9 19:34, Vlastimil Babka wrote:
>>>> On 05/09/2017 12:55 PM, zhongjiang wrote:
>>>>> From: zhong jiang <zhongjiang@huawei.com>
>>>>>
>>>>> Current, when we prepare a huge page to collapse, due to some
>>>>> reasons, it can fail to collapse. At the moment, we should
>>>>> release the preallocate huge page.
>>>>>
>>>>> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
>>>> Hmm, scratch that, there's no memory leak. The pointer to new_page is
>>>> stored in *hpage, and put_page() is called all the way up in
>>>> khugepaged_do_scan().
>>>  I see. I miss it. but why the new_page need to be release all the way.
>> AFAIK to support preallocation and reusal of preallocated page for
>> collapse attempt in different pmd. It only works for !NUMA so it's
>> likely not worth all the trouble and complicated code, so I wouldn't be
>> opposed to simplifying this.
>>
>>>  I do not see the count increment when scan success. it save the memory,
>>>  only when page fault happen.
>> I don't understand what you mean here?
>>
>>>  Thanks
>>>  zhongjiang
>>>>> ---
>>>>>  mm/khugepaged.c | 4 ++++
>>>>>  1 file changed, 4 insertions(+)
>>>>>
>>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>>> index 7cb9c88..586b1f1 100644
>>>>> --- a/mm/khugepaged.c
>>>>> +++ b/mm/khugepaged.c
>>>>> @@ -1082,6 +1082,8 @@ static void collapse_huge_page(struct mm_struct *mm,
>>>>>  	up_write(&mm->mmap_sem);
>>>>>  out_nolock:
>>>>>  	trace_mm_collapse_huge_page(mm, isolated, result);
>>>>> +	if (page != NULL && result != SCAN_SUCCEED)
>>>>> +		put_page(new_page);
>>>>>  	return;
>>>>>  out:
>>>>>  	mem_cgroup_cancel_charge(new_page, memcg, true);
>>>>> @@ -1555,6 +1557,8 @@ static void collapse_shmem(struct mm_struct *mm,
>>>>>  	}
>>>>>  out:
>>>>>  	VM_BUG_ON(!list_empty(&pagelist));
>>>>> +	if (page != NULL && result != SCAN_SUCCEED)
>>>>> +		put_page(new_page);
>>>>>  	/* TODO: tracepoints */
>>>>>  }
>>>>>  
>>>>>
>>>> .
>>>>
>>>
>>
>> .
>>
> 
> 

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