(cc on guest, DF 6.5-DEVELOPMENT, cc 8.3 [DragonFly] Release/2019-02-22)
$ cc -O2 -Wall -pthread -o /tmp/mmap_resize_uaf /tmp/mmap_resize_uaf.c
/tmp/mmap_resize_uaf.c: In function 'main':
/tmp/mmap_resize_uaf.c:131:19: warning: unused variable 'freed_page' [-Wunused-variable]
   int racefd = -1, freed_page = -1;
                   ^~~~~~~~~~
BUILD_OK
(fix module build: cd /usr/src/sys/dev/sound/sound && make  — links sound.ko incl. channel.o buffer.o; md5 patched a517bbe17032d9ba88046ab759e578ca, stock e869fcdebe0137ee26a9de85429c0baf)