Re: /proc/pid/maps containg anonymous maps that have PROT_NONE

[KOSAKI Motohiro]

> [pid 31928] mmap(NULL, 134217728, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0 <unfinished ...>
> [pid 31938] futex(0x7ffff6e489e0, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
> [pid 31928] <... mmap resumed> )        = 0x7fffe2bd3000
> 
>   which maps 7fffe2bd3000-7fffeabd3000; then
> 
> [pid 31928] munmap(0x7fffe2bd3000, 21155840 <unfinished ...>
> [pid 31938] <... futex resumed> )       = 0
> [pid 31928] <... munmap resumed> )      = 0
> 
>   which unmaps 7fffe2bd3000-7fffe4000000; and then
> 
> [pid 31928] munmap(0x7fffe8000000, 45953024) = 0
> 
>   which unmaps 7fffe8000000-7fffeabd3000.
> 
> So it's trimming off the rough edges to leave 7fffe4000000-7fffe8000000
> mapped PROT_NONE, then mprotecting what it needs of that.
> 
> Why does it mmap too much then trim it down?  Perhaps it's trying to
> minimize pagetable usage, perhaps it's internally convenient to base
> on rounded addresses, I don't know.

Yup.

glibc malloc's arena need heap size alignment.
it cause that struct arena address can calcurate "arena = any_ptr & ~(HEAP_SIZE-1)"
as old linux's "current" macro calculation.


> But the mmap is there: just easily overlooked because of the way it
> munmaps too (with strace showing hex addresses but decimal sizes).





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