[Bug 230160] linuxulator doesn't implement madvise(MADV_DONTNEED) correctly

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

            Bug ID: 230160
           Summary: linuxulator doesn't implement madvise(MADV_DONTNEED)
                    correctly
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: markj at FreeBSD.org

Our emulation of Linux's madvise(2) appears to just call sys_madvise()
directly:

{ AS(madvise_args), (sy_call_t *)sys_madvise, AUE_MADVISE, NULL, 0, 0, 0,
SY_THR_STATIC },

This sort of works for the "standard" MADV_* constants since their values are
the same on FreeBSD and Linux.  However, Linux has a somewhat surprising
behaviour for MADV_DONTNEED: if applied to a mapping of anonymous memory, the
next access will return a zero-filled page.  That is, applying MADV_DONTNEED
causes eager reclamation of pages in the range.  In contrast, our MADV_DONTNEED
implementation just causes affected pages to skip LRU, and our MADV_FREE allows
lazy reclamation of affected pages.  Some popular software, e.g., jemalloc,
expects Linux's behaviour and won't work as expected under the Linuxulator
because it just passes the parameters straight through to the native madvise(2)
implementation.

Some of Linux's other advice parameters are implemented on FreeBSD using
minherit(2) (e.g., MADV_WIPEONFORK, MADV_DONTFORK).

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