Re: [mm-unstable PATCH v4 3/9] mm/hugetlb: make pud_huge() and follow_huge_pud() aware of non-present pud entry

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/7/4 9:33, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> 
> follow_pud_mask() does not support non-present pud entry now.  As long as
> I tested on x86_64 server, follow_pud_mask() still simply returns
> no_page_table() for non-present_pud_entry() due to pud_bad(), so no severe
> user-visible effect should happen.  But generally we should call
> follow_huge_pud() for non-present pud entry for 1GB hugetlb page.
> 
> Update pud_huge() and follow_huge_pud() to handle non-present pud entries.
> The changes are similar to previous works for pud entries commit e66f17ff7177
> ("mm/hugetlb: take page table lock in follow_huge_pmd()") and commit
> cbef8478bee5 ("mm/hugetlb: pmd_huge() returns true for non-present hugepage").
> 
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> ---
> v2 -> v3:
> - fixed typos in subject and description,
> - added comment on pud_huge(),
> - added comment about fallback for hwpoisoned entry,
> - updated initial check about FOLL_{PIN,GET} flags.
> ---
>  arch/x86/mm/hugetlbpage.c |  8 +++++++-
>  mm/hugetlb.c              | 32 ++++++++++++++++++++++++++++++--
>  2 files changed, 37 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> index 509408da0da1..6b3033845c6d 100644
> --- a/arch/x86/mm/hugetlbpage.c
> +++ b/arch/x86/mm/hugetlbpage.c
> @@ -30,9 +30,15 @@ int pmd_huge(pmd_t pmd)
>  		(pmd_val(pmd) & (_PAGE_PRESENT|_PAGE_PSE)) != _PAGE_PRESENT;
>  }
>  
> +/*
> + * pud_huge() returns 1 if @pud is hugetlb related entry, that is normal
> + * hugetlb entry or non-present (migration or hwpoisoned) hugetlb entry.
> + * Otherwise, returns 0.
> + */
>  int pud_huge(pud_t pud)
>  {
> -	return !!(pud_val(pud) & _PAGE_PSE);
> +	return !pud_none(pud) &&
> +		(pud_val(pud) & (_PAGE_PRESENT|_PAGE_PSE)) != _PAGE_PRESENT;
>  }

Question: Is aarch64 supported too? It seems aarch64 version of pud_huge matches
the requirement naturally for me.

Anyway, this patch looks good to me.

Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>

Thanks.

>  
>  #ifdef CONFIG_HUGETLB_PAGE
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ad621688370b..66bb39e0fce8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6994,10 +6994,38 @@ struct page * __weak
>  follow_huge_pud(struct mm_struct *mm, unsigned long address,
>  		pud_t *pud, int flags)
>  {
> -	if (flags & (FOLL_GET | FOLL_PIN))
> +	struct page *page = NULL;
> +	spinlock_t *ptl;
> +	pte_t pte;
> +
> +	if (WARN_ON_ONCE(flags & FOLL_PIN))
>  		return NULL;
>  
> -	return pte_page(*(pte_t *)pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
> +retry:
> +	ptl = huge_pte_lock(hstate_sizelog(PUD_SHIFT), mm, (pte_t *)pud);
> +	if (!pud_huge(*pud))
> +		goto out;
> +	pte = huge_ptep_get((pte_t *)pud);
> +	if (pte_present(pte)) {
> +		page = pud_page(*pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
> +		if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
> +			page = NULL;
> +			goto out;
> +		}
> +	} else {
> +		if (is_hugetlb_entry_migration(pte)) {
> +			spin_unlock(ptl);
> +			__migration_entry_wait(mm, (pte_t *)pud, ptl);
> +			goto retry;
> +		}
> +		/*
> +		 * hwpoisoned entry is treated as no_page_table in
> +		 * follow_page_mask().
> +		 */
> +	}
> +out:
> +	spin_unlock(ptl);
> +	return page;
>  }
>  
>  struct page * __weak
>