Re: [PATCH mm-unstable v2] mm/hugetlb_vmemmap: fix memory loads ordering

[David Hildenbrand <david@redhat.com>]

On 08.01.25 08:48, 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[].
> 
> 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 | 37 +++++++++++++++++++++++++++++++++++++
>   include/linux/page_ref.h   |  2 +-
>   2 files changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 691506bdf2c5..16fa8f0cea02 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -225,11 +225,48 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page
>   	}
>   	return page;
>   }
> +
> +static __always_inline bool page_count_writable(const struct page *page, int u)
> +{
> +	if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key))
> +		return true;
> +
> +	/*
> +	 * The refcount check is ordered before the fake-head check to prevent
> +	 * the following race:
> +	 *   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[]
> +	 *
> +	 * The refcount check also prevents modification attempts to other (r/o)
> +	 * tail pages that are not fake heads.
> +	 */
> +	if (atomic_read_acquire(&page->_refcount) == u)
> +		return false;
> +
> +	return page_fixed_fake_head(page) == page;
> +}
>   #else
>   static inline const struct page *page_fixed_fake_head(const struct page *page)
>   {
>   	return page;
>   }
> +
> +static inline bool page_count_writable(const struct page *page, int u)
> +{
> +	return true;
> +}
>   #endif
>   
>   static __always_inline int page_is_fake_head(const struct page *page)
> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> index 8c236c651d1d..544150d1d5fd 100644
> --- a/include/linux/page_ref.h
> +++ b/include/linux/page_ref.h
> @@ -234,7 +234,7 @@ static inline bool page_ref_add_unless(struct page *page, int nr, int u)
>   
>   	rcu_read_lock();
>   	/* avoid writing to the vmemmap area being remapped */
> -	if (!page_is_fake_head(page) && page_ref_count(page) != u)
> +	if (page_count_writable(page, u))
>   		ret = atomic_add_unless(&page->_refcount, nr, u);
>   	rcu_read_unlock();
>   


LGTM, thanks!

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb