linux-kernel - Re: [PATCH] mm: reuse the unshared swapcache page in do_wp

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Date:   Thu, 13 Jan 2022 17:48:49 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Liang Zhang <zhangliang5@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        wangzhigang17@...wei.com
Subject: Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page

On 13.01.22 17:37, Linus Torvalds wrote:
> On Thu, Jan 13, 2022 at 6:39 AM Matthew Wilcox <willy@...radead.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