Re: [PATCH STABLE 5.10] mm/memory: add non-anonymous page check in the copy_present_page()

[Hugh Dickins]

Let me delete stable from the Cc, this discussion is not for stable.

On Thu, 27 Oct 2022, Peter Xu wrote:
> On Wed, Oct 26, 2022 at 06:48:29PM -0700, Hugh Dickins wrote:
> > 
> > And I imagined that the correct fix (short of going forward with David's
> > full changes) would be to back out to a context where one could add an
> > anon_vma_prepare(), then retry after that - involves dropping pt lock,
> > maybe gets nasty (tedious, anyway).
> 
> Right, that looks a larger changeset with minimum benefit - the page is
> still inconsistent before fork(), and also for users don't fork() at all
> after the RO pin.

Sorry, I don't understand any of what you're saying there: but you appear
to be saying ("larger changeset with minimum benefit") that my suggestion
would not be worth the effort - fair enough, but...

> 
> It looks to me Hugh's suggestion would be the best suite here for stable.
> Yuanzheng, what do you think?

... now you appear to be saying it would be worth the effort.  Oh,
perhaps you're referring to just the change to check dst anon_vma:
perhaps, but I'm really having to guess at what you mean.

But none of that matters as much as below...

> 
> For the long term I think we should wait for David's further unshare work
> so gup_must_unshare() will work for page caches too while mapped private.

I do wonder if in the long term we shall have to port all David's work
back to 5.15 and 5.10 (but I think there's yet more to come from him).
But set aside that thought for now...

More urgently, in the short term:

Peter, you've made no reference to David's mail, where he concludes that
Yuanzheng's !PageAnon patch is the appropriate one; and
David, you've made no reference to Peter's mail, where he concludes that
my doubts were correct, and it needs a different patch.

You appear to disagree over whether a RO-pinned file page needs to
be copied at fork() time.  I don't know, but I hope you can agree
on that (in the 5.10 and 5.15 context: maybe David is thinking of
the 6.0 context and Peter of the 5.10 context) before going further.

(I'm hoping David is right, and I was plain wrong, since that's easiest.)

Hugh