A pair of memory-allocation improvements in 5.13

https://en.cppreference.com/w/c/language/identifier#Reser...

The following identifiers are reserved and may not be declared in a program (doing so invokes undefined behavior):

3) All identifiers that begin with an underscore followed by a capital letter or by another underscore (these reserved identifiers allow the library to use numerous behind-the-scenes non-external macros and functions)

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The kernel has been violating this rule for a long time. It is more important in userspace though to avoid clashes with glibc internals. I recall there was once a programming guide years ago (by Rusty Russell IIRC) that suggested using the kernel's double-underscore convention in userspace, but it had to be updated once someone pointed out the above rule. If there has been a recent push to get the kernel to follow the rule, I am not aware of it.