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

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Message-Id: <20220530160223.63ae3bdef7420f252d7366ed@linux-foundation.org>
Date:   Mon, 30 May 2022 16:02:23 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] mm/swapfile: make security_vm_enough_memory_mm()
 work as expected

On Fri, 27 May 2022 17:26:24 +0800 Miaohe Lin <linmiaohe@...wei.com> wrote:

> 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.
> 
> 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.

User-visible impact?

If I'm understanding correctly, there's a risk that this fix will cause
existing setups to newly fail when attempting swapoff()?