I upgraded from WordPress 4.9.6 to 6.3 and initially everything seemed fine.
I installed the latest version of the X theme (by Themeco) with no issue.
But when I applied my old X child theme the site has crashed. The admin panel will not load, even Recovery Mode doesn't work.
I have subsequently removed all plugins and themes (except for twentytwentythree).
Nevertheless, I keep getting variations of this error in my Apache logs:
mod_fcgid: stderr: PHP Fatal error: Allowed memory size of 1073741824 bytes exhausted (tried to allocate 4096 bytes)
Out of desperation I have successfully increased the amount of memory available to 256M, then 512M and finally 1024M.
But no matter how much memory I allocate, the "tried to allocate" amount is always tiny: either 4096, 8192, or 20480 bytes.
Added to wp-config:
define( 'WP_MEMORY_LIMIT', '1024M' );
And php.ini:
memory_limit = 1024M
I have tried different versions of PHP from 7.4.33 to 8.1.23, and switching between FastCGI and FPM.
I found this comment on StackOverflow that seems to describe my situation, but I have no idea what the next step should be.
My guess is infinite recursion tried to allocate 8192 bytes typically when this is a low value that is the reason. If you up the memory above that amount and it still happens, then that's another marker for it.
PHP 8.0.30 | WordPress 6.3 | CentOS 7.9.2009