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

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Message-ID: <e2580cfa-a529-934d-861a-091c4a9714d4@redhat.com>
Date:   Thu, 20 Jan 2022 16:26:22 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "zhangliang (AG)" <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,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page

On 20.01.22 15:39, Matthew Wilcox wrote:
> On Thu, Jan 20, 2022 at 03:15:37PM +0100, David Hildenbrand wrote:
>> On 17.01.22 14:31, zhangliang (AG) wrote:
>>> Sure, I will do that :)
>>
>> I'm polishing up / testing the patches and might send something out for discussion shortly.
>> Just a note that on my branch was a version with a wrong condition that should have been fixed now.
>>
>> I am still thinking about PTE mapped THP. For these, we'll always
>> have page_count() > 1, essentially corresponding to the number of still-mapped sub-pages.
>>
>> So if we end up with a R/O mapped part of a THP, we'll always have to COW and cannot reuse ever,
>> although it's really just a single process mapping the THP via PTEs.
>>
>> One approach would be to scan the currently locked page table for entries mapping
>> this same page. If page_count() corresponds to that value, we know that only we are
>> mapping the THP and there are no additional references. That would be a special case
>> if we find an anon THP in do_wp_page(). Hm.
> 
> You're starting to optimise for some pretty weird cases at that point.

So your claim is that read-only, PTE mapped pages are weird? How do you
come to that conclusion?

If we adjust the THP reuse logic to split on additional references
(page_count() == 1) -- similarly as suggested by Linus to fix the CVE --
we're going to end up with exactly that more frequently.

> Anon THP is always going to start out aligned (and can be moved by
> mremap()).  Arguably it should be broken up if it's moved so it can be
> reformed into aligned THPs by khugepaged.

Can you elaborate, I'm missing the point where something gets moved. I
don't care about mremap() at all here.

1. You have a read-only, PTE mapped THP
2. Write fault on the THP
3. We PTE-map the THP because we run into a false positive in our COW
   logic to handle COW on PTE
4. Write fault on the PTE
5. We always have to COW each and every sub-page and can never reuse,
   because page_count() > 1

That's essentially what reuse_swap_page() tried to handle before.
Eventually optimizing for this is certainly the next step, but I'd like
to document which effect the removal of reuse_swap_page() will have to THP.

-- 
Thanks,

David / dhildenb