On 2022/6/21 15:42, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>
>> On 2022/6/21 9:35, Huang, Ying wrote:
>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>>>
>>>> On 2022/6/20 15:31, Huang, Ying wrote:
>>>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>>>>>
>>>>>> security_vm_enough_memory_mm() checks whether a process has enough memory
>>>>>> to allocate a new virtual mapping. And total_swap_pages is considered as
>>>>>> available memory while swapoff tries to make sure there's enough memory
>>>>>> that can hold the swapped out memory. But total_swap_pages contains the
>>>>>> swap space that is being swapoff. So security_vm_enough_memory_mm() will
>>>>>> success even if there's no memory to hold the swapped out memory because
>>>>>> total_swap_pages always greater than or equal to p->pages.
>>>>>
>>>>> Per my understanding, swapoff will not allocate virtual mapping by
>>>>> itself. But after swapoff, the overcommit limit could be exceeded.
>>>>> security_vm_enough_memory_mm() is used to check that. For example, in a
>>>>> system with 4GB memory and 8GB swap, and 10GB is in use,
>>>>>
>>>>> CommitLimit: 4+8 = 12GB
>>>>> Committed_AS: 10GB
>>>>>
>>>>> security_vm_enough_memory_mm() in swapoff() will fail because
>>>>> 10+8 = 18 > 12. This is expected because after swapoff, the overcommit
>>>>> limit will be exceeded.
>>>>>
>>>>> If 3GB is in use,
>>>>>
>>>>> CommitLimit: 4+8 = 12GB
>>>>> Committed_AS: 3GB
>>>>>
>>>>> security_vm_enough_memory_mm() in swapoff() will succeed because
>>>>> 3+8 = 11 < 12. This is expected because after swapoff, the overcommit
>>>>> limit will not be exceeded.
>>>>
>>>> In OVERCOMMIT_NEVER scene, I think you're right.
>>>>
>>>>>
>>>>> So, what's the real problem of the original implementation? Can you
>>>>> show it with an example as above?
>>>>
>>>> In OVERCOMMIT_GUESS scene, in a system with 4GB memory and 8GB swap, and 10GB is in use,
>>>> pages below is 8GB, totalram_pages() + total_swap_pages is 12GB, so swapoff() will succeed
>>>> instead of expected failure because 8 < 12. The overcommit limit is always *ignored* in the
>>>> below case.
>>>>
>>>> if (sysctl_overcommit_memory == OVERCOMMIT_GUESS) {
>>>> if (pages > totalram_pages() + total_swap_pages)
>>>> goto error;
>>>> return 0;
>>>> }
>>>>
>>>> Or am I miss something?
>>>
>>> Per my understanding, with OVERCOMMIT_GUESS, the number of in-use pages
>>> isn't checked at all. The only restriction is that the size of the
>>> virtual mapping created should be less than total RAM + total swap
>>
>> Do you mean the only restriction is that the size of the virtual mapping
>> *created every time* should be less than total RAM + total swap pages but
>> *total virtual mapping* is not limited in OVERCOMMIT_GUESS scene? If so,
>> the current behavior should be sane and I will drop this patch.
>
> Yes. This is my understanding.
I see. Thank you.
>
> Best Regards,
> Huang, Ying
>
>> Thanks!
>>
>>> pages. Because swapoff() will not create virtual mapping, so it's
>>> expected that security_vm_enough_memory_mm() in swapoff() always
>>> succeeds.
>>>
>>> Best Regards,
>>> Huang, Ying
>>>
>>>>
>>>> Thanks!
>>>>
>>>>>
>>>>>> In order to fix it, p->pages should be retracted from total_swap_pages
>>>>>> first and then check whether there's enough memory for inuse swap pages.
>>>>>>
>>>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>>>>
>>>>> [snip]
>>>>>
>>>>> .
>>>>>
>>>
>>> .
>>>
>
> .
>
