1. 18 Nov, 2017 1 commit
  2. 07 Nov, 2015 1 commit
  3. 04 Jun, 2014 2 commits
    • Mel Gorman's avatar
      mm: page_alloc: reduce number of times page_to_pfn is called · dc4b0caf
      Mel Gorman authored
      In the free path we calculate page_to_pfn multiple times. Reduce that.
      Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
      Acked-by: default avatarRik van Riel <riel@redhat.com>
      Cc: Johannes Weiner <hannes@cmpxchg.org>
      Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
      Cc: Jan Kara <jack@suse.cz>
      Cc: Michal Hocko <mhocko@suse.cz>
      Cc: Hugh Dickins <hughd@google.com>
      Cc: Dave Hansen <dave.hansen@intel.com>
      Cc: Theodore Ts'o <tytso@mit.edu>
      Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
      Cc: Oleg Nesterov <oleg@redhat.com>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      dc4b0caf
    • Mel Gorman's avatar
      mm: page_alloc: use word-based accesses for get/set pageblock bitmaps · e58469ba
      Mel Gorman authored
      The test_bit operations in get/set pageblock flags are expensive.  This
      patch reads the bitmap on a word basis and use shifts and masks to isolate
      the bits of interest.  Similarly masks are used to set a local copy of the
      bitmap and then use cmpxchg to update the bitmap if there have been no
      other changes made in parallel.
      
      In a test running dd onto tmpfs the overhead of the pageblock-related
      functions went from 1.27% in profiles to 0.5%.
      
      In addition to the performance benefits, this patch closes races that are
      possible between:
      
      a) get_ and set_pageblock_migratetype(), where get_pageblock_migratetype()
         reads part of the bits before and other part of the bits after
         set_pageblock_migratetype() has updated them.
      
      b) set_pageblock_migratetype() and set_pageblock_skip(), where the non-atomic
         read-modify-update set bit operation in set_pageblock_skip() will cause
         lost updates to some bits changed in the set_pageblock_migratetype().
      
      Joonsoo Kim first reported the case a) via code inspection.  Vlastimil
      Babka's testing with a debug patch showed that either a) or b) occurs
      roughly once per mmtests' stress-highalloc benchmark (although not
      necessarily in the same pageblock).  Furthermore during development of
      unrelated compaction patches, it was observed that frequent calls to
      {start,undo}_isolate_page_range() the race occurs several thousands of
      times and has resulted in NULL pointer dereferences in move_freepages()
      and free_one_page() in places where free_list[migratetype] is
      manipulated by e.g.  list_move().  Further debugging confirmed that
      migratetype had invalid value of 6, causing out of bounds access to the
      free_list array.
      
      That confirmed that the race exist, although it may be extremely rare,
      and currently only fatal where page isolation is performed due to
      memory hot remove.  Races on pageblocks being updated by
      set_pageblock_migratetype(), where both old and new migratetype are
      lower MIGRATE_RESERVE, currently cannot result in an invalid value
      being observed, although theoretically they may still lead to
      unexpected creation or destruction of MIGRATE_RESERVE pageblocks.
      Furthermore, things could get suddenly worse when memory isolation is
      used more, or when new migratetypes are added.
      
      After this patch, the race has no longer been observed in testing.
      Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
      Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
      Reported-by: default avatarJoonsoo Kim <iamjoonsoo.kim@lge.com>
      Reported-and-tested-by: default avatarVlastimil Babka <vbabka@suse.cz>
      Cc: Johannes Weiner <hannes@cmpxchg.org>
      Cc: Jan Kara <jack@suse.cz>
      Cc: Michal Hocko <mhocko@suse.cz>
      Cc: Hugh Dickins <hughd@google.com>
      Cc: Dave Hansen <dave.hansen@intel.com>
      Cc: Theodore Ts'o <tytso@mit.edu>
      Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
      Cc: Oleg Nesterov <oleg@redhat.com>
      Cc: Rik van Riel <riel@redhat.com>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Cc: <stable@vger.kernel.org>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      e58469ba
  4. 03 Jul, 2013 1 commit
  5. 10 Oct, 2012 1 commit
  6. 09 Oct, 2012 1 commit
    • Mel Gorman's avatar
      mm: compaction: cache if a pageblock was scanned and no pages were isolated · bb13ffeb
      Mel Gorman authored
      When compaction was implemented it was known that scanning could
      potentially be excessive.  The ideal was that a counter be maintained for
      each pageblock but maintaining this information would incur a severe
      penalty due to a shared writable cache line.  It has reached the point
      where the scanning costs are a serious problem, particularly on
      long-lived systems where a large process starts and allocates a large
      number of THPs at the same time.
      
      Instead of using a shared counter, this patch adds another bit to the
      pageblock flags called PG_migrate_skip.  If a pageblock is scanned by
      either migrate or free scanner and 0 pages were isolated, the pageblock is
      marked to be skipped in the future.  When scanning, this bit is checked
      before any scanning takes place and the block skipped if set.
      
      The main difficulty with a patch like this is "when to ignore the cached
      information?" If it's ignored too often, the scanning rates will still be
      excessive.  If the information is too stale then allocations will fail
      that might have otherwise succeeded.  In this patch
      
      o CMA always ignores the information
      o If the migrate and free scanner meet then the cached information will
        be discarded if it's at least 5 seconds since the last time the cache
        was discarded
      o If there are a large number of allocation failures, discard the cache.
      
      The time-based heuristic is very clumsy but there are few choices for a
      better event.  Depending solely on multiple allocation failures still
      allows excessive scanning when THP allocations are failing in quick
      succession due to memory pressure.  Waiting until memory pressure is
      relieved would cause compaction to continually fail instead of using
      reclaim/compaction to try allocate the page.  The time-based mechanism is
      clumsy but a better option is not obvious.
      Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
      Acked-by: default avatarRik van Riel <riel@redhat.com>
      Cc: Richard Davies <richard@arachsys.com>
      Cc: Shaohua Li <shli@kernel.org>
      Cc: Avi Kivity <avi@redhat.com>
      Acked-by: default avatarRafael Aquini <aquini@redhat.com>
      Cc: Fengguang Wu <fengguang.wu@intel.com>
      Cc: Michal Nazarewicz <mina86@mina86.com>
      Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
      Cc: Kyungmin Park <kyungmin.park@samsung.com>
      Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      bb13ffeb
  7. 26 Oct, 2010 1 commit
  8. 25 May, 2008 1 commit
    • Paul Jackson's avatar
      x86 boot: simplify pageblock_bits enum declaration · c801ed38
      Paul Jackson authored
      The use of #defines with '##' pre-processor concatenation is a useful
      way to form several symbol names with a common pattern.  But when there
      is just a single name obtained from that #define, it's just obfuscation.
      Better to just write the plain symbol name, as is.
      
      The following patch is a result of my wasting ten minutes looking through
      the kernel to figure out what 'PB_migrate_end' meant, and forgetting what
      I came to do, by the time I figured out that the #define PB_range macro
      defined it.
      Signed-off-by: default avatarPaul Jackson <pj@sgi.com>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      c801ed38
  9. 16 Oct, 2007 7 commits
    • KAMEZAWA Hiroyuki's avatar
      memory unplug: page isolation · a5d76b54
      KAMEZAWA Hiroyuki authored
      Implement generic chunk-of-pages isolation method by using page grouping ops.
      
      This patch add MIGRATE_ISOLATE to MIGRATE_TYPES. By this
       - MIGRATE_TYPES increases.
       - bitmap for migratetype is enlarged.
      
      pages of MIGRATE_ISOLATE migratetype will not be allocated even if it is free.
      By this, you can isolated *freed* pages from users. How-to-free pages is not
      a purpose of this patch. You may use reclaim and migrate codes to free pages.
      
      If start_isolate_page_range(start,end) is called,
       - migratetype of the range turns to be MIGRATE_ISOLATE  if
         its type is MIGRATE_MOVABLE. (*) this check can be updated if other
         memory reclaiming works make progress.
       - MIGRATE_ISOLATE is not on migratetype fallback list.
       - All free pages and will-be-freed pages are isolated.
      To check all pages in the range are isolated or not,  use test_pages_isolated(),
      To cancel isolation, use undo_isolate_page_range().
      
      Changes V6 -> V7
       - removed unnecessary #ifdef
      
      There are HOLES_IN_ZONE handling codes...I'm glad if we can remove them..
      Signed-off-by: default avatarYasunori Goto <y-goto@jp.fujitsu.com>
      Signed-off-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      a5d76b54
    • Mel Gorman's avatar
      Do not depend on MAX_ORDER when grouping pages by mobility · d9c23400
      Mel Gorman authored
      Currently mobility grouping works at the MAX_ORDER_NR_PAGES level.  This makes
      sense for the majority of users where this is also the huge page size.
      However, on platforms like ia64 where the huge page size is runtime
      configurable it is desirable to group at a lower order.  On x86_64 and
      occasionally on x86, the hugepage size may not always be MAX_ORDER_NR_PAGES.
      
      This patch groups pages together based on the value of HUGETLB_PAGE_ORDER.  It
      uses a compile-time constant if possible and a variable where the huge page
      size is runtime configurable.
      
      It is assumed that grouping should be done at the lowest sensible order and
      that the user would not want to override this.  If this is not true,
      page_block order could be forced to a variable initialised via a boot-time
      kernel parameter.
      
      One potential issue with this patch is that IA64 now parses hugepagesz with
      early_param() instead of __setup().  __setup() is called after the memory
      allocator has been initialised and the pageblock bitmaps already setup.  In
      tests on one IA64 there did not seem to be any problem with using
      early_param() and in fact may be more correct as it guarantees the parameter
      is handled before the parsing of hugepages=.
      Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
      Acked-by: default avatarAndy Whitcroft <apw@shadowen.org>
      Acked-by: default avatarChristoph Lameter <clameter@sgi.com>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      d9c23400
    • Mel Gorman's avatar
      don't group high order atomic allocations · 64c5e135
      Mel Gorman authored
      Grouping high-order atomic allocations together was intended to allow
      bursty users of atomic allocations to work such as e1000 in situations
      where their preallocated buffers were depleted.  This did not work in at
      least one case with a wireless network adapter needing order-1 allocations
      frequently.  To resolve that, the free pages used for min_free_kbytes were
      moved to separate contiguous blocks with the patch
      bias-the-location-of-pages-freed-for-min_free_kbytes-in-the-same-max_order_nr_pages-blocks.
      
      It is felt that keeping the free pages in the same contiguous blocks should
      be sufficient for bursty short-lived high-order atomic allocations to
      succeed, maybe even with the e1000.  Even if there is a failure, increasing
      the value of min_free_kbytes will free pages as contiguous bloks in
      contrast to the standard buddy allocator which makes no attempt to keep the
      minimum number of free pages contiguous.
      
      This patch backs out grouping high order atomic allocations together to
      determine if it is really needed or not.  If a new report comes in about
      high-order atomic allocations failing, the feature can be reintroduced to
      determine if it fixes the problem or not.  As a side-effect, this patch
      reduces by 1 the number of bits required to track the mobility type of
      pages within a MAX_ORDER_NR_PAGES block.
      Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
      Acked-by: default avatarAndy Whitcroft <apw@shadowen.org>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      64c5e135
    • Mel Gorman's avatar
      Bias the location of pages freed for min_free_kbytes in the same MAX_ORDER_NR_PAGES blocks · 56fd56b8
      Mel Gorman authored
      The standard buddy allocator always favours the smallest block of pages.
      The effect of this is that the pages free to satisfy min_free_kbytes tends
      to be preserved since boot time at the same location of memory ffor a very
      long time and as a contiguous block.  When an administrator sets the
      reserve at 16384 at boot time, it tends to be the same MAX_ORDER blocks
      that remain free.  This allows the occasional high atomic allocation to
      succeed up until the point the blocks are split.  In practice, it is
      difficult to split these blocks but when they do split, the benefit of
      having min_free_kbytes for contiguous blocks disappears.  Additionally,
      increasing min_free_kbytes once the system has been running for some time
      has no guarantee of creating contiguous blocks.
      
      On the other hand, CONFIG_PAGE_GROUP_BY_MOBILITY favours splitting large
      blocks when there are no free pages of the appropriate type available.  A
      side-effect of this is that all blocks in memory tends to be used up and
      the contiguous free blocks from boot time are not preserved like in the
      vanilla allocator.  This can cause a problem if a new caller is unwilling
      to reclaim or does not reclaim for long enough.
      
      A failure scenario was found for a wireless network device allocating
      order-1 atomic allocations but the allocations were not intense or frequent
      enough for a whole block of pages to be preserved for MIGRATE_HIGHALLOC.
      This was reproduced on a desktop by booting with mem=256mb, forcing the
      driver to allocate at order-1, running a bittorrent client (downloading a
      debian ISO) and building a kernel with -j2.
      
      This patch addresses the problem on the desktop machine booted with
      mem=256mb.  It works by setting aside a reserve of MAX_ORDER_NR_PAGES
      blocks, the number of which depends on the value of min_free_kbytes.  These
      blocks are only fallen back to when there is no other free pages.  Then the
      smallest possible page is used just like the normal buddy allocator instead
      of the largest possible page to preserve contiguous pages The pages in free
      lists in the reserve blocks are never taken for another migrate type.  The
      results is that even if min_free_kbytes is set to a low value, contiguous
      blocks will be preserved in the MIGRATE_RESERVE blocks.
      
      This works better than the vanilla allocator because if min_free_kbytes is
      increased, a new reserve block will be chosen based on the location of
      reclaimable pages and the block will free up as contiguous pages.  In the
      vanilla allocator, no effort is made to target a block of pages to free as
      contiguous pages and min_free_kbytes pages are scattered randomly.
      
      This effect has been observed on the test machine.  min_free_kbytes was set
      initially low but it was kept as a contiguous free block within
      MIGRATE_RESERVE.  min_free_kbytes was then set to a higher value and over a
      period of time, the free blocks were within the reserve and coalescing.
      How long it takes to free up depends on how quickly LRU is rotating.
      Amusingly, this means that more activity will free the blocks faster.
      
      This mechanism potentially replaces MIGRATE_HIGHALLOC as it may be more
      effective than grouping contiguous free pages together.  It all depends on
      whether the number of active atomic high allocations exceeds
      min_free_kbytes or not.  If the number of active allocations exceeds
      min_free_kbytes, it's worth it but maybe in that situation, min_free_kbytes
      should be set higher.  Once there are no more reports of allocation
      failures, a patch will be submitted that backs out MIGRATE_HIGHALLOC and
      see if the reports stay missing.
      
      Credit to Mariusz Kozlowski for discovering the problem, describing the
      failure scenario and testing patches and scenarios.
      
      [akpm@linux-foundation.org: cleanups]
      Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
      Acked-by: default avatarAndy Whitcroft <apw@shadowen.org>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      56fd56b8
    • Mel Gorman's avatar
      Group short-lived and reclaimable kernel allocations · e12ba74d
      Mel Gorman authored
      This patch marks a number of allocations that are either short-lived such as
      network buffers or are reclaimable such as inode allocations.  When something
      like updatedb is called, long-lived and unmovable kernel allocations tend to
      be spread throughout the address space which increases fragmentation.
      
      This patch groups these allocations together as much as possible by adding a
      new MIGRATE_TYPE.  The MIGRATE_RECLAIMABLE type is for allocations that can be
      reclaimed on demand, but not moved.  i.e.  they can be migrated by deleting
      them and re-reading the information from elsewhere.
      Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
      Cc: Andy Whitcroft <apw@shadowen.org>
      Cc: Christoph Lameter <clameter@sgi.com>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      e12ba74d
    • Mel Gorman's avatar
      Split the free lists for movable and unmovable allocations · b2a0ac88
      Mel Gorman authored
      This patch adds the core of the fragmentation reduction strategy.  It works by
      grouping pages together based on their ability to migrate or be reclaimed.
      Basically, it works by breaking the list in zone->free_area list into
      MIGRATE_TYPES number of lists.
      Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      b2a0ac88
    • Mel Gorman's avatar
      Add a bitmap that is used to track flags affecting a block of pages · 835c134e
      Mel Gorman authored
      Here is the latest revision of the anti-fragmentation patches.  Of particular
      note in this version is special treatment of high-order atomic allocations.
      Care is taken to group them together and avoid grouping pages of other types
      near them.  Artifical tests imply that it works.  I'm trying to get the
      hardware together that would allow setting up of a "real" test.  If anyone
      already has a setup and test that can trigger the atomic-allocation problem,
      I'd appreciate a test of these patches and a report.  The second major change
      is that these patches will apply cleanly with patches that implement
      anti-fragmentation through zones.
      
      kernbench shows effectively no performance difference varying between -0.2%
      and +2% on a variety of test machines.  Success rates for huge page allocation
      are dramatically increased.  For example, on a ppc64 machine, the vanilla
      kernel was only able to allocate 1% of memory as a hugepage and this was due
      to a single hugepage reserved as min_free_kbytes.  With these patches applied,
      17% was allocatable as superpages.  With reclaim-related fixes from Andy
      Whitcroft, it was 40% and further reclaim-related improvements should increase
      this further.
      
      Changelog Since V28
      o Group high-order atomic allocations together
      o It is no longer required to set min_free_kbytes to 10% of memory. A value
        of 16384 in most cases will be sufficient
      o Now applied with zone-based anti-fragmentation
      o Fix incorrect VM_BUG_ON within buffered_rmqueue()
      o Reorder the stack so later patches do not back out work from earlier patches
      o Fix bug were journal pages were being treated as movable
      o Bias placement of non-movable pages to lower PFNs
      o More agressive clustering of reclaimable pages in reactions to workloads
        like updatedb that flood the size of inode caches
      
      Changelog Since V27
      
      o Renamed anti-fragmentation to Page Clustering. Anti-fragmentation was giving
        the mistaken impression that it was the 100% solution for high order
        allocations. Instead, it greatly increases the chances high-order
        allocations will succeed and lays the foundation for defragmentation and
        memory hot-remove to work properly
      o Redefine page groupings based on ability to migrate or reclaim instead of
        basing on reclaimability alone
      o Get rid of spurious inits
      o Per-cpu lists are no longer split up per-type. Instead the per-cpu list is
        searched for a page of the appropriate type
      o Added more explanation commentary
      o Fix up bug in pageblock code where bitmap was used before being initalised
      
      Changelog Since V26
      o Fix double init of lists in setup_pageset
      
      Changelog Since V25
      o Fix loop order of for_each_rclmtype_order so that order of loop matches args
      o gfpflags_to_rclmtype uses gfp_t instead of unsigned long
      o Rename get_pageblock_type() to get_page_rclmtype()
      o Fix alignment problem in move_freepages()
      o Add mechanism for assigning flags to blocks of pages instead of page->flags
      o On fallback, do not examine the preferred list of free pages a second time
      
      The purpose of these patches is to reduce external fragmentation by grouping
      pages of related types together.  When pages are migrated (or reclaimed under
      memory pressure), large contiguous pages will be freed.
      
      This patch works by categorising allocations by their ability to migrate;
      
      Movable - The pages may be moved with the page migration mechanism. These are
      	generally userspace pages.
      
      Reclaimable - These are allocations for some kernel caches that are
      	reclaimable or allocations that are known to be very short-lived.
      
      Unmovable - These are pages that are allocated by the kernel that
      	are not trivially reclaimed. For example, the memory allocated for a
      	loaded module would be in this category. By default, allocations are
      	considered to be of this type
      
      HighAtomic - These are high-order allocations belonging to callers that
      	cannot sleep or perform any IO. In practice, this is restricted to
      	jumbo frame allocation for network receive. It is assumed that the
      	allocations are short-lived
      
      Instead of having one MAX_ORDER-sized array of free lists in struct free_area,
      there is one for each type of reclaimability.  Once a 2^MAX_ORDER block of
      pages is split for a type of allocation, it is added to the free-lists for
      that type, in effect reserving it.  Hence, over time, pages of the different
      types can be clustered together.
      
      When the preferred freelists are expired, the largest possible block is taken
      from an alternative list.  Buddies that are split from that large block are
      placed on the preferred allocation-type freelists to mitigate fragmentation.
      
      This implementation gives best-effort for low fragmentation in all zones.
      Ideally, min_free_kbytes needs to be set to a value equal to 4 * (1 <<
      (MAX_ORDER-1)) pages in most cases.  This would be 16384 on x86 and x86_64 for
      example.
      
      Our tests show that about 60-70% of physical memory can be allocated on a
      desktop after a few days uptime.  In benchmarks and stress tests, we are
      finding that 80% of memory is available as contiguous blocks at the end of the
      test.  To compare, a standard kernel was getting < 1% of memory as large pages
      on a desktop and about 8-12% of memory as large pages at the end of stress
      tests.
      
      Following this email are 12 patches that implement thie page grouping feature.
       The first patch introduces a mechanism for storing flags related to a whole
      block of pages.  Then allocations are split between movable and all other
      allocations.  Following that are patches to deal with per-cpu pages and make
      the mechanism configurable.  The next patch moves free pages between lists
      when partially allocated blocks are used for pages of another migrate type.
      The second last patch groups reclaimable kernel allocations such as inode
      caches together.  The final patch related to groupings keeps high-order atomic
      allocations.
      
      The last two patches are more concerned with control of fragmentation.  The
      second last patch biases placement of non-movable allocations towards the
      start of memory.  This is with a view of supporting memory hot-remove of DIMMs
      with higher PFNs in the future.  The biasing could be enforced a lot heavier
      but it would cost.  The last patch agressively clusters reclaimable pages like
      inode caches together.
      
      The fragmentation reduction strategy needs to track if pages within a block
      can be moved or reclaimed so that pages are freed to the appropriate list.
      This patch adds a bitmap for flags affecting a whole a MAX_ORDER block of
      pages.
      
      In non-SPARSEMEM configurations, the bitmap is stored in the struct zone and
      allocated during initialisation.  SPARSEMEM statically allocates the bitmap in
      a struct mem_section so that bitmaps do not have to be resized during memory
      hotadd.  This wastes a small amount of memory per unused section (usually
      sizeof(unsigned long)) but the complexity of dynamically allocating the memory
      is quite high.
      
      Additional credit to Andy Whitcroft who reviewed up an earlier implementation
      of the mechanism an suggested how to make it a *lot* cleaner.
      Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
      Cc: Andy Whitcroft <apw@shadowen.org>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      835c134e