linux-kernel - Re: [PATCH resend v2] /dev/zero: try to align PMD

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Message-ID: <eeac8523-801a-4b9a-81f4-6539ce1955c2@redhat.com>
Date: Wed, 30 Jul 2025 11:33:52 +0200
From: David Hildenbrand <david@...hat.com>
To: Zhang Qilong <zhangqilong3@...wei.com>, arnd@...db.de,
 gregkh@...uxfoundation.org
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 lorenzo.stoakes@...cle.com, wangkefeng.wang@...wei.com, sunnanyong@...wei.com
Subject: Re: [PATCH resend v2] /dev/zero: try to align PMD_SIZE for private
 mapping

On 30.07.25 11:19, Zhang Qilong wrote:
> Attempt to map aligned to huge page size for private mapping which
> could achieve performance gains, the mprot_tw4m in libMicro average
> execution time on arm64:
>    - Test case:        mprot_tw4m
>    - Before the patch:   22 us
>    - After the patch:    17 us
> 
> Signed-off-by: Zhang Qilong <zhangqilong3@...wei.com>
> ---
> v2:
> - Add comments on code suggested by Lorenzo
> - Use IS_ENABLED to check THP config
> 
>   drivers/char/mem.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 48839958b0b1..c27cc89bd02d 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -525,11 +525,18 @@ static unsigned long get_unmapped_area_zero(struct file *file,
>   		 * so as not to confuse shmem with our handle on "/dev/zero".
>   		 */
>   		return shmem_get_unmapped_area(NULL, addr, len, pgoff, flags);
>   	}
>   
> -	/* Otherwise flags & MAP_PRIVATE: with no shmem object beneath it */
> +	/*
> +	 * Otherwise flags & MAP_PRIVATE: with no shmem object beneath it,
> +	 * attempt to map aligned to huge page size if possible, otherwise we
> +	 * fall back to system page size mappings.
> +	 */
> +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> +		return thp_get_unmapped_area(file, addr, len, pgoff, flags);
> +
>   	return mm_get_unmapped_area(current->mm, file, addr, len, pgoff, flags);
>   #else
>   	return -ENOSYS;
>   #endif
>   }

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb