add kernel feature wishlist

KERNEL_FEATURE_WISHLIST.md

@@ -0,0 +1,29 @@
+Very important and should be an easy sell:
+
+* improved robustness for high vma count on high memory machines
+* much higher `vm.max_map_count` by default
+* work on improving performance and resource usage with high vma count
+* add a way to disable the brk heap and have mmap grow upwards like it did in
+  the past (preserving the same high base entropy)
+
+Somewhat important and an easy sell:
+
+* mremap flag to disable unmapping the source mapping
+    * also needed by jemalloc for different reasons
+    * not needed if the kernel gets first class support for arbitrarily sized
+      guard pages and a virtual memory quarantine feature
+
+Fairly infeasible to land but could reduce overhead and extend coverage of
+security features to other code directly using mmap:
+
+* first class support for arbitrarily sized guard pages for mmap and mremap to
+  eliminate half of the resulting VMAs and reduce 2 system calls to 1
+    * not usable if it doesn't support mremap (shrink, grow, grow via move)
+    * not usable if the guard page size is static
+    * should support changing guard size for mremap growth via move
+    * must be possible to set it up from the process
+* virtual memory quarantine
+    * must be possible to set it up from the process
+* first-class support for aligned mappings with mmap and ideally mremap
+    * not usable unless guard page support is provided and of course it has to
+      work with this too