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Date:   Thu, 20 Jan 2022 16:39:55 +0100
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Subject: Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page
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To:     Matthew Wilcox <willy@infradead.org>
Cc:     "zhangliang (AG)" <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,
        Linus Torvalds <torvalds@linux-foundation.org>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 20.01.22 16:36, Matthew Wilcox wrote:
> On Thu, Jan 20, 2022 at 04:26:22PM +0100, David Hildenbrand wrote:
>> 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?
> 
> Because normally anon THP pages are PMD mapped.  That's rather
> the point of anon THPs.

For example unless we are talking about *drumroll* COW handling.

> 
>> 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.
> 
> I don't understand.  Are we talking past each other?  As I understand
> the situation we're talking about here, a process has created a THP,
> done something to cause it to be partially mapped (or mapped in a
> misaligned way) in its own address space, then forked, and we're
> trying to figure out if it's safe to reuse it?  I say that situation is
> rare enough that it's OK to always allocate an order-0 page and
> copy into it.

Yes, we are talking past each other and no, I am talking about fully
mapped THP, just mapped via PTEs.

Please refer to our THP COW logic: do_huge_pmd_wp_page()

> 
>>> 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.
> 
> I'm talking about step 0.  How do we get a read-only, PTE-mapped THP?
> Through mremap() or perhaps through an mprotect()/mmap()/munmap() that
> failed to split the THP.

do_huge_pmd_wp_page()

-- 
Thanks,

David / dhildenb