[Bug 249067] coredumps include whole maps

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=249067

            Bug ID: 249067
           Summary: coredumps include whole maps
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: brooks at FreeBSD.org

FreeBSD coredumps dump whole apps even when most pages haven't been touched. 
Modern allocators often allocate very large regions of VA e.g. with 1GiB-100GiB
mmaps rather than making repeated calls to mmap. When such processes dump core
the whole map gets dumped even though it's nearly all unmapped pages.  Linux
breaks such maps into a set of PT_LOADs where unbacked anonymous pages aren't
present in the file.

We ran into this on a very slow riscv (100MHz FPGA with very slow disk), but we
believe it effects everyone.

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