linux-kernel - Re: [PATCH v2 4/5] mm/hugetlb: use PMD page lock to protect CONT-PTE entries

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Message-ID: <3661415c-6069-24ed-b647-6fe6993bddae@redhat.com>
Date:   Tue, 23 Aug 2022 10:14:50 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        akpm@...ux-foundation.org, songmuchun@...edance.com,
        mike.kravetz@...cle.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/5] mm/hugetlb: use PMD page lock to protect CONT-PTE
 entries

On 23.08.22 09:50, Baolin Wang wrote:
> Considering the pmd entries of a CONT-PMD hugetlb can not span on
> multiple PMDs, we can change to use the PMD page lock, which can
> be much finer grain that lock in the mm.
> 
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>  include/linux/hugetlb.h | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 3a96f67..d4803a89 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -892,9 +892,17 @@ static inline gfp_t htlb_modify_alloc_mask(struct hstate *h, gfp_t gfp_mask)
>  static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
>  					   struct mm_struct *mm, pte_t *pte)
>  {
> -	VM_BUG_ON(huge_page_size(h) == PAGE_SIZE);
> +	unsigned long hp_size = huge_page_size(h);
>  
> -	if (huge_page_size(h) == PMD_SIZE) {
> +	VM_BUG_ON(hp_size == PAGE_SIZE);
> +
> +	/*
> +	 * Considering CONT-PMD size hugetlb, since the CONT-PMD entry
> +	 * can not span multiple PMDs, then we can use the fine grained
> +	 * PMD page lock.
> +	 */
> +	if (hp_size == PMD_SIZE ||
> +	    (hp_size > PMD_SIZE && hp_size < PUD_SIZE)) {
>  		return pmd_lockptr(mm, (pmd_t *) pte);
>  	} else if (huge_page_size(h) < PMD_SIZE) {
>  		unsigned long mask = ~(PTRS_PER_PTE * sizeof(pte_t) - 1);

Is there a measurable performance gain? IOW, do we really care?

-- 
Thanks,

David / dhildenb