Most notable to the mm pull request is the enabling of full ASLR randomization for 32-bit programs. Yes, about Address Space Layout Randomization. If you're not familiar with it, see Wikipedia.
The enabling of full randomization on i386 and x86_32 was done by Hector Marco-Gisbert. He explained in the patch:
Currently on i386 and on X86_64 when emulating X86_32 in legacy mode, only the stack and the executable are randomized but not other mmapped files (libraries, vDSO, etc.). This patch enables randomization for the libraries, vDSO and mmap requests on i386 and in X86_32 in legacy mode.The MM pull request in full can be found here. Besides this big improvement for 32-bit x86 ASLR, there is also Kernel Address Sanitizer (KASAN) improvements, fixed mmiotrace for huge pages, minimal INVPCI support, and other work.
By default on i386 there are 8 bits for the randomization of the libraries, vDSO and mmaps which only uses 1MB of VA.
This patch preserves the original randomness, using 1MB of VA out of 3GB or 4GB. We think that 1MB out of 3GB is not a big cost for having the ASLR.
The first obvious security benefit is that all objects are randomized (not only the stack and the executable) in legacy mode which highly increases the ASLR effectiveness, otherwise the attackers may use these non-randomized areas. But also sensitive setuid/setgid applications are more secure because currently, attackers can disable the randomization of these applications by setting the ulimit stack to "unlimited". This is a very old and widely known trick to disable the ASLR in i386 which has been allowed for too long.
Another trick used to disable the ASLR was to set the ADDR_NO_RANDOMIZE personality flag, but fortunately this doesn't work on setuid/setgid applications because there is security checks which clear Security-relevant flags.
This patch always randomizes the mmap_legacy_base address, removing the possibility to disable the ASLR by setting the stack to "unlimited".