linux-kernel - Re: [PATCH v3 6/7] mm: make clear_huge_page cache clear only around the fault address

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 for Android: free password hash cracker in your pocket

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

Date:	Thu, 16 Aug 2012 18:16:47 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Andi Kleen <ak@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Alex Shi <alex.shu@...el.com>,
	Jan Beulich <jbeulich@...ell.com>,
	Robert Richter <robert.richter@....com>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-mips@...ux-mips.org,
	linux-sh@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v3 6/7] mm: make clear_huge_page cache clear only around
 the fault address

Hi Kirill,

On Thu, Aug 16, 2012 at 06:15:53PM +0300, Kirill A. Shutemov wrote:
>  	for (i = 0; i < pages_per_huge_page;
>  	     i++, p = mem_map_next(p, page, i)) {

It may be more optimal to avoid a multiplication/shiftleft before the
add, and to do:

  	for (i = 0, vaddr = haddr; i < pages_per_huge_page;
  	     i++, p = mem_map_next(p, page, i), vaddr += PAGE_SIZE) {

>  		cond_resched();
> -		clear_user_highpage(p, addr + i * PAGE_SIZE);
> +		vaddr = haddr + i*PAGE_SIZE;

Not sure if gcc can optimize it away because of the external calls.

> +		if (!ARCH_HAS_USER_NOCACHE || i == target)
> +			clear_user_highpage(page + i, vaddr);
> +		else
> +			clear_user_highpage_nocache(page + i, vaddr);
>  	}


My only worry overall is if there can be some workload where this may
actually slow down userland if the CPU cache is very large and
userland would access most of the faulted in memory after the first
fault.

So I wouldn't mind to add one more check in addition of
!ARCH_HAS_USER_NOCACHE above to check a runtime sysctl variable. It'll
waste a cacheline yes but I doubt it's measurable compared to the time
it takes to do a >=2M hugepage copy.

Furthermore it would allow people to benchmark its effect without
having to rebuild the kernel themself.

All other patches looks fine to me.

Thanks!
Andrea
--
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/