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

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Date:   Mon, 20 Jun 2022 20:12:27 +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 2022/6/20 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?

> 
> 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]
> 
> .
>