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

[David Hildenbrand]

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@xxxxxxxxxx>
> Closes: https://lore.kernel.org/20241128142028.GA3506@willie-the-truck/
> Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> ---
>   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.

--
Cheers,

David / dhildenb