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: <1402923474.3958.34.camel@debian>
Date:	Mon, 16 Jun 2014 20:57:54 +0800
From:	Chen Yucong <slaoub@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	mhocko@...e.cz, hannes@...xchg.org, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmscan.c: avoid recording the original scan targets
 in shrink_lruvec()

On Mon, 2014-06-09 at 21:27 +0800, Chen Yucong wrote:
> Via https://lkml.org/lkml/2013/4/10/334 , we can find that recording the
> original scan targets introduces extra 40 bytes on the stack. This patch
> is able to avoid this situation and the call to memcpy(). At the same time,
> it does not change the relative design idea.
> 
> ratio = original_nr_file / original_nr_anon;
> 
> If (nr_file > nr_anon), then ratio = (nr_file - x) / nr_anon.
>  x = nr_file - ratio * nr_anon;
> 
> if (nr_file <= nr_anon), then ratio = nr_file / (nr_anon - x).
>  x = nr_anon - nr_file / ratio;
> 
Hi Andrew Morton,

I think the patch
 
[PATCH]
mm-vmscanc-avoid-recording-the-original-scan-targets-in-shrink_lruvec-fix.patch

which I committed should be discarded. Because It have some critical
defects.
    1) If we want to solve the divide-by-zero and unfair problems, it
needs to two variables for recording the ratios.
 
    2) For "x = nr_file - ratio * nr_anon", the "x" is likely to be a
negative number. we can assume:

      nr[LRU_ACTIVE_FILE] = 30
      nr[LRU_INACTIVE_FILE] = 30
      nr[LRU_ACTIVE_ANON] = 0
      nr[LRU_INACTIVE_ANON] = 40

      ratio = 60/40 = 3/2

When the value of (nr_reclaimed < nr_to_reclaim) become false, there are
the following results:
      nr[LRU_ACTIVE_FILE] = 15
      nr[LRU_INACTIVE_FILE] = 15
      nr[LRU_ACTIVE_ANON] = 0
      nr[LRU_INACTIVE_ANON] = 25
 
      nr_file = 30
      nr_anon = 25

      x = 30 - 25 * (3/2) = 30 - 37.5 = -7.5.

The result is too terrible. 
   
   3) This method is less accurate than the original, especially for the
qualitative difference between FILE and ANON that is very small.

thx!
cyc  
   

--
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/

Powered by blists - more mailing lists

Powered by Openwall GNU/*/Linux Powered by OpenVZ