Apparently, the segfaults were not jemalloc's fault.

dist/mastodon-sidekiq.service

@@ -9,7 +9,7 @@ WorkingDirectory=/home/mastodon/live
 Environment="RAILS_ENV=production"
 Environment="DB_POOL=25"
 Environment="MALLOC_ARENA_MAX=2"
-Environment="LD_PRELOAD=libmimalloc.so"
+Environment="LD_PRELOAD=libjemalloc.so"
 ExecStart=/home/mastodon/.rbenv/shims/bundle exec sidekiq -c 25
 TimeoutSec=15
 Restart=always

dist/mastodon-streaming.service

@@ -9,6 +9,7 @@ WorkingDirectory=/home/mastodon/live
 Environment="NODE_ENV=production"
 Environment="PORT=4000"
 Environment="STREAMING_CLUSTER_NUM=1"
+Environment="LD_PRELOAD=libjemalloc.so"
 ExecStart=/usr/bin/node ./streaming
 TimeoutSec=15
 Restart=always

dist/mastodon-web.service

@@ -8,7 +8,7 @@ User=mastodon
 WorkingDirectory=/home/mastodon/live
 Environment="RAILS_ENV=production"
 Environment="PORT=3000"
-Environment="LD_PRELOAD=libmimalloc.so"
+Environment="LD_PRELOAD=libjemalloc.so"
 ExecStart=/home/mastodon/.rbenv/shims/bundle exec puma -C config/puma.rb
 ExecReload=/bin/kill -SIGUSR1 $MAINPID
 TimeoutSec=15