mm: fix khugepaged activation policy

Since the introduction of mTHP, the docuementation has stated that
khugepaged would be enabled when any mTHP size is enabled, and disabled
when all mTHP sizes are disabled.  There are 2 problems with this; 1. 
this is not what was implemented by the code and 2.  this is not the
desirable behavior.

Desirable behavior is for khugepaged to be enabled when any PMD-sized THP
is enabled, anon or file.  (Note that file THP is still controlled by the
top-level control so we must always consider that, as well as the PMD-size
mTHP control for anon).  khugepaged only supports collapsing to PMD-sized
THP so there is no value in enabling it when PMD-sized THP is disabled. 
So let's change the code and documentation to reflect this policy.

Further, per-size enabled control modification events were not previously
forwarded to khugepaged to give it an opportunity to start or stop. 
Consequently the following was resulting in khugepaged eroneously not
being activated:

  echo never > /sys/kernel/mm/transparent_hugepage/enabled
  echo always > /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled

[[email protected]: v3]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ryan Roberts <[email protected]>
Fixes: 3485b88390b0 ("mm: thp: introduce multi-size THP sysfs interface")
Closes: https://lore.kernel.org/linux-mm/[email protected]/
Acked-by: David Hildenbrand <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Lance Yang <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions