Re: [PATCH 1/2] mm/memory_hotplug: remove head page reference in do_migrate_range

[Matthew Wilcox]

On Mon, Jan 23, 2023 at 01:08:49PM -0800, Sidhartha Kumar wrote:
> On 1/23/23 12:37 PM, Matthew Wilcox wrote:
> > On Mon, Jan 23, 2023 at 12:23:46PM -0800, Sidhartha Kumar wrote:
> > > @@ -1637,14 +1637,13 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
> > >   			continue;
> > >   		page = pfn_to_page(pfn);
> > >   		folio = page_folio(page);
> > > -		head = &folio->page;
> > > -		if (PageHuge(page)) {
> > > -			pfn = page_to_pfn(head) + compound_nr(head) - 1;
> > > +		if (folio_test_hugetlb(folio)) {
> > > +			pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1;
> > >   			isolate_hugetlb(folio, &source);
> > >   			continue;
> > > -		} else if (PageTransHuge(page))
> > > -			pfn = page_to_pfn(head) + thp_nr_pages(page) - 1;
> > > +		} else if (folio_test_transhuge(folio))
> > > +			pfn = folio_pfn(folio) + thp_nr_pages(page) - 1;
> > 
> > I'm pretty sure those two lines should be...
> > 
> > 		} else if (folio_test_large(folio) > 			pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1;
> > 
> > But, erm ... we're doing this before we have a refcount on the page,
> > right?  So this is unsafe because the page might change which folio
> > it is in.  And the folio we found earlier might become a tail page
> > of a different folio.  (As the comment below explains, HWPoison pages
> > won't, so it's not unsafe for them).
> > 
> 
> Thanks for the explanation of why this is unsafe. Would it be worth to put
> this code block inside the
> 
> 		if (!get_page_unless_zero(page))
> 			continue;
> 
> 		put_page(page);
> 
> block found lower? My motivation for this series is the HPageMigratable call
> in patch 2 is the last user of the huge page flag test macros so a
> conversion would allow for the removal of the macro. I thought I could also
> remove the head page references found in this function, but if that would
> cause too much churn in a complicated sub-system it can be dropped.

I think we just have to be very careful when working without a page ref.

Now, specifically to the matter of converting HPageMigratable(), I think
that's fine.  Your folio_test_hugetlb_##flname macro does not have a
VM_BUG_ON_PGFLAGS(PageTail(page), page) in it, unlike folio_flags().
So it looks like even if your folio becomes a tail page in the middle
of scan_movable_pages(), you won't hit a BUG().

Now, should we go further?  Possibly.  But I'm more concerned that we
haven't really figured out which functions should be checking this.
Maybe we should drop the BUG entirely and rely more on the type system
(and people not casting) to prevent errors.

We could go a lot further with the type system and define a new type for
"might be a folio but we don't have a refcount on it".  But we don't
do a lot of work with unreferenced folios, so I'm not inclined to go to
all that effort.

Perhaps we want a folio_maybe_X() series of functions that don't warn
if the folio has morphed into not-a-folio.  I don't know.