Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

[David Hildenbrand]

On 17.12.21 22:36, Linus Torvalds wrote:
> On Fri, Dec 17, 2021 at 12:55 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> If we have a shared anonymous page we cannot have GUP references, not
>> even R/O ones. Because GUP would have unshared and copied the page,
>> resulting in a R/O mapped anonymous page.
> 
> Doing a GUP on an actual shared page is wrong to begin with.
> 
> You even know that, you try to use "page_mapcount() > 1" to disallow it.

GUP is incomaptible with shared anonymous pages, therefore it has to
trigger unsharing, correct.

> 
> My point is that it's wrong regardless, and that "mapcount" is
> dubious, and that COW cannot - and must not - use mapcount, and that I
> think your shared case should strive to avoid it for the exact same
> reason.

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

We can protect from the transtition == 1 -> >1 using the mmap_lock.

For COW the mapcount is the only thing that matters *if we take GUP* out
of the equation. And that's exactly what we

OTOH, take a look which issues resulted from the page_count changes.
That's what I call dubious, sorry to say.

> 
> So, what I think should happen is:
> 
>  (a) GUP makes sure that it only ever looks up pages that can be
> shared with this VM. This may in involve breaking COW early with any
> past fork().

Is that unsharing as we propose it?

> 
>  (b) it marks such pages so that any future work will not cause them
> to COW either

Right, exactly. GUP before fork does not result in a page getting shared
again.

> 
> Note that (a) is not necessarily "always COW and have to allocate and
> copy new page". In particular, if the page is already writable, you
> know you already have exclusive access to it and don't need to COW.
> 
> And if it isn't writable, then the other common case is "the cow has
> only one user, and it's us" - that's the "refcount == 1" case.
> 
> And (b) is what we do with that page_maybe_dma_pinned() logic for
> fork(), but also for things like swap cache creation (eg see commit
> feb889fb40fa: "mm: don't put pinned pages into the swap cache").

I fully agree with b). GUP before fork is a totally different set of
problems than GUP after fork.

> 
> Note that this code all already exists, and already works - even
> without getting the (very expensive) mmap_sem. So it works with
> fast-GUP and it can race with concurrent forking by another thread,
> which is why we also have that seqcount thing.

I know, I studied it intensively :)

> 
> As far as I can tell, your "mapcount" logic fundamentally requires
> mmap_sem for the fork() race avoidance, for example.

Yes. Or any other more lightweight synchronization in the future. For
now this is just perfect.

> 
> So this is why I don't like the mapcount games - I think they are very
> fragile, and not at all as logical as the two simple rules a/b above.

I don't really see anything fragile, really. I'm happy to learn as always.

> 
> I believe you can make mapcount games _work_ - we used to have
> something like that. It was incredibly fragile, and it had its own set
> of bugs, but with enough care it's doable.

We made it work, and it was comparatively simple.

-- 
Thanks,

David / dhildenb