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Running AlmaLinux 9.2 and MariaDB 10.11.

Trying to use in my.cnf:

tmpdir = /dev/shm

This works fine when PHP-FPM is connecting using TCP port.

Now trying to use faster Socket (/var/lib/mysql/mysql.sock) from PHP-FPM.

This doesn't work out of the box, but had to apply a workaround mentioned in https://jira.mariadb.org/browse/MDEV-30520:

chcon -t mysqld_exec_t /usr/sbin/mariadbd

After that MariaDB won't start, because it cannot access /dev/shm.

Then allowed access to tmpfs file context using mysqld_tmpfs.te policy per https://mariadb.com/kb/en/selinux/#allowing-access-to-the-tmpfs-file-context:

module mysqld_tmpfs 1.0;

require {
    type tmpfs_t;
    type mysqld_t;
    class dir { write search read remove_name open getattr add_name };
    class file { write getattr read lock create unlink open };
}

allow mysqld_t tmpfs_t:dir { write search read remove_name open getattr add_name };

allow mysqld_t tmpfs_t:file { write getattr read lock create unlink open };

MariaDB now runs, but when it tries to create temp files seeing the following errors using ausearch -c 'mariadbd' --raw:

type=AVC msg=audit(1690215382.357:58492): avc:  denied  { map } for  pid=602807 comm="mariadbd" path="/dev/shm/#sql-temptable-932b7-22f-62.MAD" dev="tmpfs" ino=1183 scontext=system_u:system_r:mysqld_t:s0 tcontext=system_u:object_r:tmpfs_t:s0 tclass=file permissive=0 
type=SYSCALL msg=audit(1690215382.357:58492): arch=c000003e syscall=9 success=no exit=-13 a0=0 a1=7 a2=3 a3=4001 items=0 ppid=1 pid=602807 auid=4294967295 uid=986 gid=985 euid=986 suid=986 fsuid=986 egid=985 sgid=985 fsgid=985 tty=(none) ses=4294967295 comm="mariadbd" exe="/usr/sbin/mariadbd" subj=system_u:system_r:mysqld_t:s0 key=(null)ARCH=x86_64 SYSCALL=mmap AUID="unset" UID="mysql" GID="mysql" EUID="mysql" SUID="mysql" FSUID="mysql" EGID="mysql" SGID="mysql" FSGID="mysql"
type=PROCTITLE msg=audit(1690215382.357:58492): proctitle="/usr/sbin/mariadbd"

or in human readable form:

SELinux is preventing /usr/sbin/mariadbd from map access on the file /dev/shm/#sql-temptable-932b7-22f-62.MAD.

*****  Plugin restorecon_source (92.2 confidence) suggests   *****************

If you want to fix the label. 
/usr/sbin/mariadbd default label should be bin_t.
Then you can run restorecon.
Do
# /sbin/restorecon -v /usr/sbin/mariadbd

*****  Plugin catchall_boolean (7.83 confidence) suggests   ******************

If you want to allow domain to can mmap files
Then you must tell SELinux about this by enabling the 'domain_can_mmap_files' boolean.

Do
setsebool -P domain_can_mmap_files 1

*****  Plugin catchall (1.41 confidence) suggests   **************************

If you believe that mariadbd should be allowed map access on the #sql-temptable-932b7-22f-62.MAD file by default.
Then you should report this as a bug.
You can generate a local policy module to allow this access.
Do
allow this access for now by executing:
# ausearch -c 'mariadbd' --raw | audit2allow -M my-mariadbd
# semodule -X 300 -i my-mariadbd.pp

Please advise.

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  • Putting tmpdir into /dev/shm seems very wrong. Why do you do that? Jul 24, 2023 at 17:45
  • Seeing "Temporary tables created on disk: 40% (108K on disk / 268K total)" even though tmp_table_size = 384M and max_heap_table_size = 384M is in place. It's a large WordPress database, so trying to find the cause there also. In the meantime would like to store temp files in memory instead on disk.
    – Ivan
    Jul 24, 2023 at 18:15
  • why? Is there benchmarks that suggest that approach? Things that mariadb/InnoDB can do in RAM, it does on RAM. All you gain by putting temporary tablespace into RAM is that you make your Linux flush out page cache earlier due to memory pressure from tmpfs when you need it the most. Jul 24, 2023 at 18:17
  • After switching to socket instead of TCP port and commenting out tmpdir = /dev/shm editors started complaining things are slower.
    – Ivan
    Jul 24, 2023 at 18:21
  • as said: I doubt putting tmpdir on a tmpfs is even remotely a good idea. Could you explain to me why you come to the conclusion that it is? Jul 24, 2023 at 18:21

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