ksm: the mm interface to ksm

This patch presents the mm interface to a dummy version of ksm.c, for
better scrutiny of that interface: the real ksm.c follows later.

When CONFIG_KSM is not set, madvise(2) reject MADV_MERGEABLE and
MADV_UNMERGEABLE with EINVAL, since that seems more helpful than
pretending that they can be serviced.  But when CONFIG_KSM=y, accept them
even if KSM is not currently running, and even on areas which KSM will not
touch (e.g.  hugetlb or shared file or special driver mappings).

Like other madvices, report ENOMEM despite success if any area in the
range is unmapped, and use EAGAIN to report out of memory.

Define vma flag VM_MERGEABLE to identify an area on which KSM may try
merging pages: leave it to ksm_madvise() to decide whether to set it.
Define mm flag MMF_VM_MERGEABLE to identify an mm which might contain
VM_MERGEABLE areas, to minimize callouts when forking or exiting.

Based upon earlier patches by Chris Wright and Izik Eidus.

Signed-off-by: Hugh Dickins <[email protected]>
Signed-off-by: Chris Wright <[email protected]>
Signed-off-by: Izik Eidus <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Wu Fengguang <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Cc: Avi Kivity <[email protected]>
Cc: Nick Piggin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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