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Date:   Fri, 17 Dec 2021 23:43:44 +0100
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To:     Linus Torvalds <torvalds@linux-foundation.org>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
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On 17.12.21 23:18, Linus Torvalds wrote:
> On Fri, Dec 17, 2021 at 1:47 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> For now I have not heard a compelling argument why the mapcount is
>> dubious, I repeat:
>>
>> * mapcount can only increase due to fork()
>> * mapcount can decrease due to unmap / zap
> 
> And to answer the "why is this dubious", let' sjust look at your
> actual code that I reacted to:
> 
> +       vmf->page = vm_normal_page(vmf->vma, vmf->address, vmf->orig_pte);
> +       if (vmf->page && PageAnon(vmf->page) && !PageKsm(vmf->page) &&
> +           page_mapcount(vmf->page) > 1) {
> 
> Note how you don't just check page_mapcount(). Why not? Because
> mapcount is completely immaterial if it's not a PageAnon page, so you
> test for that.
> 
> So even when you do the mapcount read as one atomic thing, it's one
> atomic thing that depends on _other_ things, and all these checks are
> not atomic.
> 
> But a PageAnon() page can actually become a swap-backed page, and as
> far as I can tell, your code doesn't have any locking to protect
> against that.

The pages stay PageAnon(). swap-backed pages simply set a bit IIRC.
mapcount still applies.

> 
> So now you need not only the mmap_sem (to protect against fork), you
> also need the page lock (to protect against rmap changing the type of
> page).

No, I don't think so. But I'm happy to be proven wrong because I might
just be missing something important.

> 
> I don't see you taking the page lock anywhere. Maybe the page table
> lock ends up serializing sufficiently with the rmap code that it ends
> up working
> 
> In the do_wp_page() path, we currently do those kinds of racy checks
> too, but then we do a trylock_page, and re-do them. And at any time
> there is any question about things, we fall back to copying - because
> a copy is always safe.

Yes, I studied that code in detail as well.

> 
> Well, it's always safe if we have the rule that "once we've pinned
> things, we don't cause them to be COW again".

We should also be handling FOLL_GET, but that's a completely different
discussion.

> 
> But that "it's safe if" was exactly my (b) case.
> 
> That's why I much prefer the model I'm trying to push - it's
> conceptually quite simple. I can literally explain mine at a
> conceptual level with that "break pre-existing COW, make sure no
> future COW" model.

:)

We really might be talking about the same thing just that my point is
that the mapcount is the right thing to use for making the discussion
whether to break COW -> triger unsharing.

> 
> In contrast, I look at your page_mapcount() code, and I go "there is
> no conceptual rules here, and the actual implementation details look
> dodgy".
> 
> I personally like having clear conceptual rules - as opposed to random
> implementation details.

Oh, don't get me wrong, me to. But for me it just all makes perfect.

What we document is:

"The fault is an unsharing request to unshare a shared anonymous page
(-> mapped R/O). Does not apply to KSM."

And the code checks for exactly that. And in that context the mapcount
just expresses exactly what we want. Again, unless I am missing
something important that you raise above.


Anyhow, it's late in Germany. thanks for the discussion Linus!

-- 
Thanks,

David / dhildenb