QResource: add support for duplicating mapped memory on Linux

This makes QResourceFileEngine avoid allocating heap memory in the
implementation of QFile::map() calls with MapPrivateOption. We do that
by asking the kernel to duplicate the mapping to the same memory pages,
which increases our virtual memory space but doesn't initially take up
any more RAM.

We have to be careful to ensure the resulting mapping is copy-on-write
and that this setting is independent of the original map's. The former
is easy: the mremap() system call with MREMAP_DONTUNMAP only works on
private memory blocks, so by definition any private + writable block is
copy-on-write.

The latter is slightly more difficult: if the original block was already
writable, then the kernel could give us a new map to the exact same
pages[*], which isn't exactly helpful. Therefore, we need to check if
the QResourceRoot points to a private, read-only block.

[*] in those conditions, mremap() is documented to replace the original
    mapping with a zero page, which means we have a near certainty of
    crashing, instead of accidentally, silently corrupting memory.

Change-Id: I6979d02a7395405cbf23fffd17c90fb1f1612ed7
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

0 files changed, 0 insertions, 0 deletions