linux-kernel - Re: [RFC PATCH] futex: replace get_user_pages() with get_user_pages

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4E035785.6050504@linux.intel.com>
Date:	Thu, 23 Jun 2011 08:11:01 -0700
From:	Darren Hart <dvhart@...ux.intel.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	tglx@...utronix.de, linux-kernel@...r.kernel.org, mingo@...e.hu,
	a.p.zijlstra@...llo.nl
Subject: Re: [RFC PATCH] futex: replace get_user_pages() with get_user_pages_fast()



On 06/22/2011 11:21 PM, KOSAKI Motohiro wrote:
> Hello,
> 
> Is there any reason to take mmap_sem explicitly here?
> Should we change gup_fast() to allow NULL argument (ie for avoid get_page)?
> 
> ============================
>   down_read(&mm->mmap_sem);
>   get_user_pages(current, current->mm)
>   up_read(&mm->mmap_sem);
> 
> and
> 
>   get_user_pages_fast()
> 
> make an equivalent result, And latter would be better when mamp_sem
> highly contended case, because it can avoid to take mmap_sem if
> the target page doesn't need a page fault.


I can't speak authoritatively here, but it seems to me that
get_user_pages_fast falls back to get_user_pages with mmap_sem anyway,
so this seems like a reasonable optimization for the best case, with a
minor overhead for the slow case.

Peter, am I missing something? Is there a reason you left this as is
during your fast gup futex update?

> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> ---
>  kernel/futex.c |    8 +++-----
>  1 files changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index fe28dc2..9f7b1ae 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -351,13 +351,11 @@ static inline void put_futex_key(union futex_key *key)
>   */
>  static int fault_in_user_writeable(u32 __user *uaddr)
>  {
> -	struct mm_struct *mm = current->mm;
>  	int ret;
> +	struct page *page;
> 
> -	down_read(&mm->mmap_sem);
> -	ret = get_user_pages(current, mm, (unsigned long)uaddr,
> -			     1, 1, 0, NULL, NULL);
> -	up_read(&mm->mmap_sem);
> +	ret = get_user_pages_fast((unsigned long)uaddr, 1, 1, &page);
> +	put_page(page);
> 
>  	return ret < 0 ? ret : 0;
>  }

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/