linux-kernel - Re: [PATCH next] hugetlb: pte_alloc_huge() to replace huge pte_alloc

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Date:   Thu, 20 Apr 2023 10:08:13 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <muchun.song@...ux.dev>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH next] hugetlb: pte_alloc_huge() to replace huge
 pte_alloc_map()

On 04/18/23 22:22, Hugh Dickins wrote:
> Some architectures can have their hugetlb pages down at the lowest PTE
> level: their huge_pte_alloc() using pte_alloc_map(), but without any
> following pte_unmap().  Since none of these arches uses CONFIG_HIGHPTE,
> this is not seen as a problem at present; but would become a problem if
> forthcoming changes were to add an rcu_read_lock() into pte_offset_map(),
> with the rcu_read_unlock() expected in pte_unmap().
> 
> Similarly in their huge_pte_offset(): pte_offset_kernel() is good enough
> for that, but it's probably less confusing if we define pte_offset_huge()
> along with pte_alloc_huge().  Only define them without CONFIG_HIGHPTE:
> so there would be a build error to signal if ever more work is needed.
> 
> For ease of development, define these now for 6.4-rc1, ahead of any use:
> then architectures can integrate patches using them, independent from mm.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
> 
>  include/linux/hugetlb.h | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)

Hate to say that I am ignorant of the 'forthcoming changes' that might
add an rcu_read_lock() into pte_offset_map().  But, the reasoning is
sound and code is fine.

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>

-- 
Mike Kravetz

> 
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -183,6 +183,23 @@ extern struct list_head huge_boot_pages;
>  
>  /* arch callbacks */
>  
> +#ifndef CONFIG_HIGHPTE
> +/*
> + * pte_offset_huge() and pte_alloc_huge() are helpers for those architectures
> + * which may go down to the lowest PTE level in their huge_pte_offset() and
> + * huge_pte_alloc(): to avoid reliance on pte_offset_map() without pte_unmap().
> + */
> +static inline pte_t *pte_offset_huge(pmd_t *pmd, unsigned long address)
> +{
> +	return pte_offset_kernel(pmd, address);
> +}
> +static inline pte_t *pte_alloc_huge(struct mm_struct *mm, pmd_t *pmd,
> +				    unsigned long address)
> +{
> +	return pte_alloc(mm, pmd) ? NULL : pte_offset_huge(pmd, address);
> +}
> +#endif
> +
>  pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
>  			unsigned long addr, unsigned long sz);
>  /*