[Bug 230160] linuxulator doesn't implement madvise(MADV_DONTNEED) and any MADV_ flags with values >= 8 correctly

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230160

--- Comment #12 from commit-hook at freebsd.org ---
A commit references this bug:

Author: markj
Date: Thu Jun 25 20:30:31 UTC 2020
New revision: 362631
URL: https://svnweb.freebsd.org/changeset/base/362631

Log:
  Implement an approximation of Linux MADV_DONTNEED semantics.

  Linux MADV_DONTNEED is not advisory: it has side effects for anonymous
  memory, and some system software depends on that.  In particular,
  MADV_DONTNEED causes anonymous pages to be discarded.  If the mapping is
  a private mapping of a named object then subsequent faults are to
  repopulate the range from that object, otherwise pages will be
  zero-filled.  For mappings of non-anonymous objects, Linux MADV_DONTNEED
  can be implemented in the same way as our MADV_DONTNEED.

  This implementation differs from Linux semantics in its handling of
  private mappings, inherited through fork(), of non-anonymous objects.
  After applying MADV_DONTNEED, subsequent faults will repopulate the
  mapping from the parent object rather than the root of the shadow chain.

  PR:           230160
  Reviewed by:  alc, kib
  Sponsored by: The FreeBSD Foundation
  MFC after:    2 weeks
  Differential Revision:        https://reviews.freebsd.org/D25330

Changes:
  head/sys/compat/linux/linux_mmap.c

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