linux-kernel - Re: [PATCH v2 1/3] mm/swapfile: make security_vm_enough_memory

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Date:   Tue, 21 Jun 2022 16:20:17 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     "Huang, Ying" <ying.huang@...el.com>
CC:     <akpm@...ux-foundation.org>, <david@...hat.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] mm/swapfile: make security_vm_enough_memory_mm()
 work as expected

On 2022/6/21 15:42, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@...wei.com> writes:
> 
>> On 2022/6/21 9:35, Huang, Ying wrote:
>>> Miaohe Lin <linmiaohe@...wei.com> writes:
>>>
>>>> On 2022/6/20 15:31, Huang, Ying wrote:
>>>>> Miaohe Lin <linmiaohe@...wei.com> 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@...wei.com>
>>>>>
>>>>> [snip]
>>>>>
>>>>> .
>>>>>
>>>
>>> .
>>>
> 
> .
>