Re: [PATCH mm-unstable v1] mm/hugetlb_vmemmap: fix memory loads ordering

[Yu Zhao <yuzhao@google.com>]

On Tue, Jan 7, 2025 at 1:49 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 07.01.25 05:35, Yu Zhao wrote:
> > Using x86_64 as an example, for a 32KB struct page[] area describing a
> > 2MB hugeTLB, HVO reduces the area to 4KB by the following steps:
> > 1. Split the (r/w vmemmap) PMD mapping the area into 512 (r/w) PTEs;
> > 2. For the 8 PTEs mapping the area, remap PTE 1-7 to the page mapped
> >     by PTE 0, and at the same time change the permission from r/w to
> >     r/o;
> > 3. Free the pages PTE 1-7 used to map, hence the reduction from 32KB
> >     to 4KB.
> >
> > However, the following race can happen due to improperly memory loads
> > ordering:
> >    CPU 1 (HVO)                     CPU 2 (speculative PFN walker)
> >
> >    page_ref_freeze()
> >    synchronize_rcu()
> >                                    rcu_read_lock()
> >                                    page_is_fake_head() is false
> >    vmemmap_remap_pte()
> >    XXX: struct page[] becomes r/o
> >
> >    page_ref_unfreeze()
> >                                    page_ref_count() is not zero
> >
> >                                    atomic_add_unless(&page->_refcount)
> >                                    XXX: try to modify r/o struct page[]
> >
> > Specifically, page_is_fake_head() must be ordered after
> > page_ref_count() on CPU 2 so that it can only return true for this
> > case, to avoid the later attempt to modify r/o struct page[].
>
> I *think* this is correct.
>
> >
> > This patch adds the missing memory barrier and makes the tests on
> > page_is_fake_head() and page_ref_count() done in the proper order.
> >
> > Fixes: bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with speculative PFN walkers")
> > Reported-by: Will Deacon <will@kernel.org>
> > Closes: https://lore.kernel.org/20241128142028.GA3506@willie-the-truck/
> > Signed-off-by: Yu Zhao <yuzhao@google.com>
> > ---
> >   include/linux/page-flags.h | 2 +-
> >   include/linux/page_ref.h   | 8 ++++++--
> >   2 files changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 691506bdf2c5..6b8ecf86f1b6 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -212,7 +212,7 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page
> >        * cold cacheline in some cases.
> >        */
> >       if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
> > -         test_bit(PG_head, &page->flags)) {
> > +         test_bit_acquire(PG_head, &page->flags)) {
>
> This change will affect all page_fixed_fake_head() users, like ordinary
> PageTail even on !hugetlb.
>
> I assume you want an explicit memory barrier in the single problematic
> caller instead.

Let me make it HVO specific in v2. It might look cleaner that way.