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Message-ID: <46e94dee-188f-48a2-800d-30a05a31d57e@lucifer.local>
Date: Wed, 30 Jul 2025 14:04:37 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Zhang Qilong <zhangqilong3@...wei.com>
Cc: arnd@...db.de, gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, david@...hat.com, wangkefeng.wang@...wei.com,
sunnanyong@...wei.com
Subject: Re: [PATCH resend v2] /dev/zero: try to align PMD_SIZE for private
mapping
On Wed, Jul 30, 2025 at 05:19:05PM +0800, 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.
> + */
Nit, but put a space after this as it aplies to both blocks.
> + if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> + return thp_get_unmapped_area(file, addr, len, pgoff, flags);
As mentioned by Matthew, and as requested originally, please make this an #ifdef
/ #else.
You might then want to factor out the #ifdef CONFIG_MMU (gross) at a higher
level.
I'm sick of us bending over backwards for museum piece (or should be) nommu but
anyway.
Something like
#ifdef CONFIG_MMU
static unsigned long get_unmapped_area_zero(struct file *file,
unsigned long addr, unsigned long len,
unsigned long pgoff, unsigned long flags)
{
return -ENOSYS;
}
#else
...
It's ugly but unfortunately I think necessary.
> +
> return mm_get_unmapped_area(current->mm, file, addr, len, pgoff, flags);
> #else
> return -ENOSYS;
> #endif
> }
> --
> 2.43.0
>
Thanks, Lorenzo
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