Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page

From: David Hildenbrand <david@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Matthew Wilcox <willy@infradead.org>
Cc: Liang Zhang <zhangliang5@huawei.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	wangzhigang17@huawei.com
Subject: Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page
Date: Thu, 13 Jan 2022 17:48:49 +0100	[thread overview]
Message-ID: <172ccfbb-7e24-db21-7d84-8c8d8c3805fd@redhat.com> (raw)
In-Reply-To: <CAHk-=wjB0i-B=U-DhpAajQx3f6bp1X==neLOrg0jwq29mgz=3g@mail.gmail.com>

On 13.01.22 17:37, Linus Torvalds wrote:
> On Thu, Jan 13, 2022 at 6:39 AM Matthew Wilcox <willy@infradead.org> wrote:
>>
>> Let's bring Linus in on this, but I think this reintroduces all of the
>> mapcount problems that we've been discussing recently.
>>
>> How about this as an alternative?
> 
> No, at that point reuse_swap_page() is the better thing to do.
> 
> Don't play games with page_count() (or even worse games with
> swap_count). The page count is only stable if it's 1. Any other value
> means that it can fluctuate due to concurrent lookups, some of which
> can be done locklessly under RCU.

I'm pretty sure the patch as is will reintroduce the CVE. So I think in
addition to the reuse_swap_page() check we need more.

I'm wondering if we can get rid of the mapcount checks in
reuse_swap_page() and instead check for page_count() and swapcount only.

We don't care if it's unstable in a sense than it will be bigger than
what we expect. In that case we COW as we would already do.

Thoughts?

-- 
Thanks,

David / dhildenb