Add "todo" directory.

Following an idea raised during our last discussion on TODOs, I committed
a new directory "todo/", and one file in it, todo/mm, with TODOs and ideas
for memory management.

I suggest that we allow free commits to this directory - with no overhead
of "review" of these informal todo items.

If we decide we don't like this format, we can easily move the content of
these files to some other tool or database or whatever.

todo/mm

+1. Lazy population
+==================
+
+Currently, an anonymous mmap() allocates physical memory pages for the
+entire specified memory range.
+
+This is similar to MAP_POPULATE in Linux, but is not the default on Linux.
+Rather, its default is lazy population: Physical memory is only allocated
+to pages when they are actually used. The benefit of this can be noticed
+in various places, but one particularly interesting benefit is to threads:
+Each threads mmap()s a 1MB (by default) stack. In OSV's eager population,
+a full 1MB of physical memory is allocated to each thread. With lazy
+population, if a thread only uses 10 KB of its stack (which is not unlikely!),
+only about this much of physical memory is actually allocated.
+
+So OSV should also have lazy population.
+
+Some implementation ideas:
+
+1. Continue to use both small and huge pages, and lazily allocate both.
+   I.e., a 1MB stack will get a page fault on every new 4K used, but a
+   huge 500MB allocation (e.g., Java's heap) will get page faults that
+   clear 2MB at a time.
+
+2. For small pages, consider this approach: Allocate one zeroed 4K page,
+   call it page_zero. New mmaps() will be filled with ptes pointing
+   to this page_zero, all marked read-only. If the program reads these
+   addresses, it gets 0 as expected. When the program writes to such
+   an address, we get a page fault. When the page-fault handler sees the
+   fault is at an address pointing to page_zero, it allocates a new physical
+   page, zeros it ("copy on write") and sets the PTE to point to it (TODO:
+   do we need TLB flush here?).
+
+3. For huge pages, especially 1G pages, allocating an unused zero page is a
+   waste. Instead, use an arbitrary page address, and mark it non-present
+   (not readable, and not writable). On write fault, allocate a new zero
+   huge page as before. On read fault, also allocate a zero huge page
+   if we haven't done this before, but remember who's using it so we can
+   soon "expire" this page and mark all its references not-present again
+   (this will definitely require a TLB flush).
+
+Of course, look also at what Linux does here to get ideas.
+
+
+2. mmap() writeback
+===================
+
+Currently, mmap()ing a file reads it into memory once, but if one writes
+to this memory area, the file is NOT written to - not sponteneously and
+not even if you call msync().
+
+This is similar to MAP_PRIVATE in Linux, but is not what people usually
+have in mind when they use mmap. Java has an interface memory-mapping of
+files (see MappedByteBuffer), and its user do expect that data written
+into such mapped memory will be written back the backing file.