MariaDB crashed: Unknown/unsupported storage engine: InnoDB

I’ve a Debian GNU/Linux 9 (4GB, 2 CPUs) on Digitalocean. Tonight (I’ve done nothing) my DB (mariaDB) crashed with this errors. I ran a wordpress with InnoDB and myISAM tables:

2020-10-17  0:51:18 140430430813568 [Note] InnoDB: Using mutexes to ref count buffer pool pages
2020-10-17  0:51:18 140430430813568 [Note] InnoDB: The InnoDB memory heap is disabled
2020-10-17  0:51:18 140430430813568 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2020-10-17  0:51:18 140430430813568 [Note] InnoDB: GCC builtin __atomic_thread_fence() is used for memory barrier
2020-10-17  0:51:18 140430430813568 [Note] InnoDB: Compressed tables use zlib 1.2.8
2020-10-17  0:51:18 140430430813568 [Note] InnoDB: Using Linux native AIO
2020-10-17  0:51:18 140430430813568 [Note] InnoDB: Using SSE crc32 instructions
2020-10-17  0:51:18 140430430813568 [Note] InnoDB: Initializing buffer pool, size = 500.0M
InnoDB: mmap(549126144 bytes) failed; errno 12
2020-10-17  0:51:18 140430430813568 [ERROR] InnoDB: Cannot allocate memory for the buffer pool
2020-10-17  0:51:18 140430430813568 [ERROR] Plugin 'InnoDB' init function returned error.
2020-10-17  0:51:18 140430430813568 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
2020-10-17  0:51:18 140430430813568 [Note] Plugin 'FEEDBACK' is disabled.
2020-10-17  0:51:18 140430430813568 [ERROR] **Unknown/unsupported storage engine: InnoDB**
2020-10-17  0:51:18 140430430813568 [ERROR] Aborting

My full DB conf:

#
# These groups are read by MariaDB server.
# Use it for options that only the server (but not clients) should see
#
# See the examples of server my.cnf files in /usr/share/mysql/
#

# this is read by the standalone daemon and embedded servers
[server]

# this is only for the mysqld standalone daemon
[mysqld]

#
# * Basic Settings
#
user        = mysql
pid-file    = /var/run/mysqld/mysqld.pid
socket      = /var/run/mysqld/mysqld.sock
port        = 3306
basedir     = /usr
datadir     = /var/lib/mysql
tmpdir      = /tmp
lc-messages-dir = /usr/share/mysql
skip-external-locking

# Instead of skip-networking the default is now to listen only on
# localhost which is more compatible and is not less secure.
bind-address        = 127.0.0.1

#
# * Fine Tuning
#
key_buffer_size     = 16M
max_allowed_packet  = 16M
thread_stack        = 192K
thread_cache_size       = 8
# This replaces the startup script and checks MyISAM tables if needed
# the first time they are touched
myisam_recover_options  = BACKUP
#max_connections        = 100
#table_cache            = 64
#thread_concurrency     = 10

innodb_buffer_pool_instances = 1
innodb_buffer_pool_size = 500M
max_heap_table_size     = 25M
tmp_table_size          = 25M
#log_slow_queries        = /var/log/mysql/mysql-slow.log
#long_query_time = 2
#log-queries-not-using-indexes

#
# * Query Cache Configuration
#
query_cache_limit   = 2M
query_cache_size        = 50M

#
# * Logging and Replication
#
# Both location gets rotated by the cronjob.
# Be aware that this log type is a performance killer.
# As of 5.1 you can enable the log at runtime!
#general_log_file        = /var/log/mysql/mysql.log
#general_log             = 1
#
# Error log - should be very few entries.
#
log_error = /var/log/mysql/error.log
#
# Enable the slow query log to see queries with especially long duration
#slow_query_log_file    = /var/log/mysql/mariadb-slow.log
#long_query_time = 10
#log_slow_rate_limit    = 1000
#log_slow_verbosity = query_plan
#log-queries-not-using-indexes
#
# The following can be used as easy to replay backup logs or for replication.
# note: if you are setting up a replication slave, see README.Debian about
#       other settings you may need to change.
#server-id      = 1
#log_bin            = /var/log/mysql/mysql-bin.log
expire_logs_days    = 10
max_binlog_size   = 100M
#binlog_do_db       = include_database_name
#binlog_ignore_db   = exclude_database_name

#
# * InnoDB
#
# InnoDB is enabled by default with a 10MB datafile in /var/lib/mysql/.
# Read the manual for more InnoDB related options. There are many!

#
# * Security Features
#
# Read the manual, too, if you want chroot!
# chroot = /var/lib/mysql/
#
# For generating SSL certificates you can use for example the GUI tool "tinyca".
#
# ssl-ca=/etc/mysql/cacert.pem
# ssl-cert=/etc/mysql/server-cert.pem
# ssl-key=/etc/mysql/server-key.pem
#
# Accept only connections using the latest and most secure TLS protocol version.
# ..when MariaDB is compiled with OpenSSL:
# ssl-cipher=TLSv1.2
# ..when MariaDB is compiled with YaSSL (default in Debian):
# ssl=on

#
# * Character sets
#
# MySQL/MariaDB default is Latin1, but in Debian we rather default to the full
# utf8 4-byte character set. See also client.cnf
#
character-set-server  = utf8mb4
collation-server      = utf8mb4_general_ci

#
# * Unix socket authentication plugin is built-in since 10.0.22-6
#
# Needed so the root database user can authenticate without a password but
# only when running as the unix root user.
#
# Also available for other users if required.
# See https://mariadb.com/kb/en/unix_socket-authentication-plugin/

# this is only for embedded server
[embedded]

# This group is only read by MariaDB servers, not by MySQL.
# If you use the same .cnf file for MySQL and MariaDB,
# you can put MariaDB-only options here
[mariadb]

# This group is only read by MariaDB-10.1 servers.
# If you use the same .cnf file for MariaDB of different versions,
# use this group for options that older servers don't understand
[mariadb-10.1]

Here my first page of htop

Could not export/copy htop result.

I would be happy if you could help me out!!

Thanks a lot
M

InnoDB: mmap(549126144 bytes) failed; errno 12

Above error seems database is not getting enough memory as per configuration. Can you stop php-fpm first then try to start database server.