mmap: move doc to README

README.adoc

@@ -3442,6 +3442,44 @@ Bibliography:
 * https://stackoverflow.com/questions/2264384/how-do-i-use-ioctl-to-manipulate-my-kernel-module/44613896#44613896
 * https://askubuntu.com/questions/54239/problem-with-ioctl-in-a-simple-kernel-module/926675#926675
 
+==== mmap
+
+In guest:
+
+....
+/mmap.sh
+echo $?
+....
+
+Outcome: the test passes:
+
+....
+0
+....
+
+Sources:
+
+* link:kernel_module/mmap.c[]
+* link:kernel_module/user/mmap.c[]
+* link:rootfs_overlay/mmap.sh[]
+
+The `mmap` system call allows us to share memory between user and kernel space without copying.
+
+In this example, we make a tiny 4 byte kernel buffer available to user-space, and we then modify it on userspace, and check that the kernel can see the modification.
+
+`mmap`, like most more complex <<file-operations>>, does not work with <<debugfs>> as of 4.9, so we use a <<procfs>> file for it.
+
+Example adapted from: https://coherentmusings.wordpress.com/2014/06/10/implementing-mmap-for-transferring-data-from-user-space-to-kernel-space/
+
+Bibliography:
+
+* https://stackoverflow.com/questions/10760479/mmap-kernel-buffer-to-user-space/10770582#10770582
+* https://stackoverflow.com/questions/1623008/allocating-memory-for-user-space-from-kernel-thread
+* https://stackoverflow.com/questions/6967933/mmap-mapping-in-user-space-a-kernel-buffer-allocated-with-kmalloc
+* https://github.com/jeremytrimble/ezdma
+* https://github.com/simonjhall/dma
+* https://github.com/ikwzm/udmabuf
+
 ==== Character devices
 
 In guest: