Re: MM patches against 2.5.31

["Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>]

On Wed, Aug 21, 2002 at 07:29:04PM -0700, Andrew Morton wrote:
> 
> I've uploaded a rollup of pending fixes and feature work
> against 2.5.31 to
> 
> http://www.zip.com.au/~akpm/linux/patches/2.5/2.5.31/2.5.31-mm1/
> 
> The rolled up patch there is suitable for ongoing testing and
> development.  The individual patches are in the broken-out/
> directory and should all be documented.

Sorry, but we still have the page release race in multiple places.
Look at the following (page starts with page_count == 1):

Processor 1                          Processor 2
refill_inactive: lines 378-395
   as page count == 1 we'll
   continue with line 401

                                     __pagevec_release: line 138
				       calls release_pages
				     release_pages: line 100-111
				       put_page_test_zero brings the
				       page count to 0 and we'll continue
				       at line 114. Note that this may
				       happen while another processor holds
				       the lru lock, i.e. there is no
				       point in checking for page count == 0
				       with the lru lock held because
				       the lru lock doesn't protect against
				       decrements of page count after
				       the check.
  line 401: page_cache_get
  resurrects the page, page
  count is now 1.
  lines 402-448.
  line 448 calls __pagevec_release

__pagevec_release: line 138
  calls release_pages
release_pages: lines 100-111
  put_page_test_zero brings the
  page count back to 0 (!!!)
  i.e. we continue at line 114:

  lines 114-123.
  The page count == 0 check in line
  123 is successful and the page
  is returned to the buddy allocator
  
				       lines 114-123.
				       The page count == 0 check in line
				       123 is successful, i.e. the page
				       is returned to the buddy allocator
				       a second time. ===> BOOM


Neither the lru lock nor any of the page count == 0 checks can
prevent this from happening.

    regards   Christian

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