Re: [PATCH v2 1/5] mm/hugetlb: fix races when looking up a CONT-PTE size hugetlb page

[David Hildenbrand]

On 23.08.22 09:50, Baolin Wang wrote:
> On some architectures (like ARM64), it can support CONT-PTE/PMD size
> hugetlb, which means it can support not only PMD/PUD size hugetlb
> (2M and 1G), but also CONT-PTE/PMD size(64K and 32M) if a 4K page size
> specified.
> 
> So when looking up a CONT-PTE size hugetlb page by follow_page(), it
> will use pte_offset_map_lock() to get the pte entry lock for the CONT-PTE
> size hugetlb in follow_page_pte(). However this pte entry lock is incorrect
> for the CONT-PTE size hugetlb, since we should use huge_pte_lock() to
> get the correct lock, which is mm->page_table_lock.
> 
> That means the pte entry of the CONT-PTE size hugetlb under current
> pte lock is unstable in follow_page_pte(), we can continue to migrate
> or poison the pte entry of the CONT-PTE size hugetlb, which can cause
> some potential race issues, and following pte_xxx() validation is also
> unstable in follow_page_pte(), even though they are under the 'pte lock'.
> 
> Moreover we should use huge_ptep_get() to get the pte entry value of
> the CONT-PTE size hugetlb, which already takes into account the subpages'
> dirty or young bits in case we missed the dirty or young state of the
> CONT-PTE size hugetlb.
> 
> To fix above issues, introducing a new helper follow_huge_pte() to look
> up a CONT-PTE size hugetlb page, which uses huge_pte_lock() to get the
> correct pte entry lock to make the pte entry stable, as well as
> supporting non-present pte handling.
> 
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> ---
>  include/linux/hugetlb.h |  8 ++++++++
>  mm/gup.c                | 11 ++++++++++
>  mm/hugetlb.c            | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 72 insertions(+)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 3ec981a..d491138 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -207,6 +207,8 @@ struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address,
>  struct page *follow_huge_pd(struct vm_area_struct *vma,
>  			    unsigned long address, hugepd_t hpd,
>  			    int flags, int pdshift);
> +struct page *follow_huge_pte(struct vm_area_struct *vma, unsigned long address,
> +			     pmd_t *pmd, int flags);
>  struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>  				pmd_t *pmd, int flags);
>  struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address,
> @@ -312,6 +314,12 @@ static inline struct page *follow_huge_pd(struct vm_area_struct *vma,
>  	return NULL;
>  }
>  
> +static inline struct page *follow_huge_pte(struct vm_area_struct *vma,
> +				unsigned long address, pmd_t *pmd, int flags)
> +{
> +	return NULL;
> +}
> +
>  static inline struct page *follow_huge_pmd(struct mm_struct *mm,
>  				unsigned long address, pmd_t *pmd, int flags)
>  {
> diff --git a/mm/gup.c b/mm/gup.c
> index 3b656b7..87a94f5 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -534,6 +534,17 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>  	if (unlikely(pmd_bad(*pmd)))
>  		return no_page_table(vma, flags);
>  
> +	/*
> +	 * Considering PTE level hugetlb, like continuous-PTE hugetlb on
> +	 * ARM64 architecture.
> +	 */
> +	if (is_vm_hugetlb_page(vma)) {
> +		page = follow_huge_pte(vma, address, pmd, flags);
> +		if (page)
> +			return page;
> +		return no_page_table(vma, flags);
> +	}
> +
>  	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
>  	pte = *ptep;
>  	if (!pte_present(pte)) {
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6c00ba1..cf742d1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6981,6 +6981,59 @@ struct page * __weak
>  	return NULL;
>  }
>  
> +/* Support looking up a CONT-PTE size hugetlb page. */
> +struct page * __weak
> +follow_huge_pte(struct vm_area_struct *vma, unsigned long address,
> +		pmd_t *pmd, int flags)
> +{
> +	struct mm_struct *mm = vma->vm_mm;
> +	struct hstate *hstate = hstate_vma(vma);
> +	unsigned long size = huge_page_size(hstate);
> +	struct page *page = NULL;
> +	spinlock_t *ptl;
> +	pte_t *ptep, pte;
> +
> +	/*
> +	 * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> +	 * follow_hugetlb_page().
> +	 */
> +	if (WARN_ON_ONCE(flags & FOLL_PIN))
> +		return NULL;
> +
> +	ptep = huge_pte_offset(mm, address, size);
> +	if (!ptep)
> +		return NULL;
> +
> +retry:
> +	ptl = huge_pte_lock(hstate, mm, ptep);
> +	pte = huge_ptep_get(ptep);
> +	if (pte_present(pte)) {
> +		page = pte_page(pte);
> +		if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
> +			page = NULL;
> +			goto out;
> +		}
> +	} else {
> +		if (!(flags & FOLL_MIGRATION)) {
> +			page = NULL;
> +			goto out;
> +		}
> +
> +		if (is_hugetlb_entry_migration(pte)) {
> +			spin_unlock(ptl);
> +			__migration_entry_wait_huge(ptep, ptl);
> +			goto retry;
> +		}
> +		/*
> +		 * hwpoisoned entry is treated as no_page_table in
> +		 * follow_page_mask().
> +		 */
> +	}
> +out:
> +	spin_unlock(ptl);
> +	return page;
> +}
> +
>  struct page * __weak
>  follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>  		pmd_t *pmd, int flags)


Can someone explain why:
* follow_page() goes via follow_page_mask() for hugetlb
* __get_user_pages() goes via follow_hugetlb_page() and never via
  follow_page_mask() for hugetlb?

IOW, why can't we make follow_page_mask() just not handle hugetlb and
route everything via follow_hugetlb_page() -- we primarily only have to
teach it to not trigger faults.


What's the reason that this hugetlb code has to be overly complicated?

-- 
Thanks,

David / dhildenb