git: kernel - Fix a race and enable the VM read shortcut feature by default

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxxxxx>]

commit 74d299cb1b84453ab71490ab2c85e2dfec664a4b
Author: Matthew Dillon <dillon@apollo.backplane.com>
Date:   Mon Feb 18 12:08:29 2013 -0800

    kernel - Fix a race and enable the VM read shortcut feature by default
    
    * Fix a lookup/access race.  No known cases hit the race but decided
      it needed to be fixed for safety.
    
      Instead of looking up and holding the VM page we know try to busy it,
      and only access the content if we are able to do so non-blocking.
      This costs a bit more in overhead but handles the page more properly.
    
      /usr/obj/usr/src
      time tar cf /dev/null .
    
      0.734u 5.781s 0:06.51 100.0%    24+66k 0+0io 0pf+0w	(shorcut disabled)
      0.664u 2.382s 0:03.05 99.6%     24+66k 0+0io 0pf+0w	(shorcut enabled)
    
    * Default vm.read_shortcut_enable to 1.  The feature is now enabled by
      default.
    
    * The feature has been in the tree a while default disabled and needs wider
      use, so it is being enabled by default.  The feature is only useful on
      64-bit systems (i.e. so the DMAP can be used).  It allows the buffer
      cache and the VM page mapping code to be completely bypassed in situations
      where the file data is available in the VM page cache.

Summary of changes:
 sys/kern/vfs_helper.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/74d299cb1b84453ab71490ab2c85e2dfec664a4b


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