linux-kernel - Re: [PATCH v4 15/20] powerpc/mm: Avoid useless lock with single page fragments

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Message-Id: <78cb82d6-1a13-150d-9daa-021bd2f98e7a@linux.ibm.com>
Date:   Wed, 19 Sep 2018 08:26:26 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        aneesh.kumar@...ux.vnet.ibm.com
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 15/20] powerpc/mm: Avoid useless lock with single page
 fragments

On 9/18/18 10:27 PM, Christophe Leroy wrote:
> There is no point in taking the page table lock as
> pte_frag is always NULL when we have only one fragment.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
>   arch/powerpc/mm/pgtable-frag.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c
> index bc924822dcd6..ab4910e92aaf 100644
> --- a/arch/powerpc/mm/pgtable-frag.c
> +++ b/arch/powerpc/mm/pgtable-frag.c
> @@ -85,6 +85,9 @@ static pte_t *get_pte_from_cache(struct mm_struct *mm)
>   {
>   	void *pte_frag, *ret;
> 
> +	if (PTE_FRAG_NR == 1)
> +		return NULL;
> +
>   	spin_lock(&mm->page_table_lock);
>   	ret = mm->context.pte_frag;
>   	if (ret) {
> 

May be update get_pmd_from_cache too?

-aneesh