Re: [PATCHv3, RESEND 5/8] khugepaged: Allow to collapse a page shared across fork

[John Hubbard]

On 2020-04-14 14:35, Kirill A. Shutemov wrote:
> On Mon, Apr 13, 2020 at 01:48:22PM -0700, John Hubbard wrote:
>
> [Thanks for all your suggestions and corrections]
>
> > > +	if (IS_ENABLED(CONFIG_DEBUG_VM) && expected_refcount > refcount) {
> > > +		pr_err("expected_refcount: %d, refcount: %d\n",
> > > +				expected_refcount, refcount);
> > > +		dump_page(page, "Unexpected refcount");
> >
> >
> > I see two issues with the pr_err() and the dump_page() call:
> >
> > 1. You probably want to rate limit this, otherwise you'll have a big
> > problem if lots of pages are pinned!
>
> Nope. Only if kernel is buggy. See below.
>
> > 2. Actually, I don't think you'd want to print anything at all here, even with
> > rate limiting, because doing so presumes that "unexpected" means "wrong". And I
> > think this patch doesn't expect to have GUP pins (or pin_user_pages() pins, ha),
> > but that doesn't mean that they're wrong to have.
>
> See condition. We only do it if refcount is *below* expected refcount. It
> should never happen. Pinned page would have refcount above expected.

Yes, you are right. I misread the condition. This actually is just right. :)

thanks,
--
John Hubbard
NVIDIA