xfs_io: allow mmap command to reserve some free space

xfs_io: allow mmap command to reserve some free space

When using xfs_io to rewrite LTP's mmap16 testcase for xfstests,
we always hit mremap ENOMEM error. But according to linux commit:

  90a8020 vfs: fix data corruption when blocksize < pagesize for mmaped data

The reproducer shouldn't use MREMAP_MAYMOVE flag for mremap(). So we
need a stable method to make mremap can extend space from the original
mapped starting address.

Generally if we try to mremap from the original mapped starting point
in a C program, at first we always do:

  addr = mmap(NULL, res_size, prot, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  munmap(addr, res_size);

Then do:

  addr = mmap(addr, real_len, ...);

The "res_size" is bigger than "real_len". This will help us get a
region between real_len and res_size, which maybe free space. The
truth is we can't guarantee that this free memory will stay free.
But this method is still very helpful for reserve some free space
in short time.

This patch bring in the "res_size" by use a new -s option. If don't use
this option, xfs_io -c "mmap 0 1024" -c "mremap 8192" will hit ENOMEM
error nearly 100%, but if use this option, it nearly always remap
successfully.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>