Messages in this thread | | | Date | Fri, 17 Dec 2021 23:43:44 +0100 | From | David Hildenbrand <> | 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
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