Re: AMAP_SHARED (was Re: XIP)

[yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi)]

hi,

> A few more comments:
> 
> - AMAP_SHARED itself is a fine concept; it's used by shared memory.
> 
>       sys/kern/sysv_shm.c:
>           452         error = uvm_map(&vm->vm_map, &attach_va, size, uobj, 0, 
> 0,
>           453             UVM_MAPFLAG(prot, prot, UVM_INH_SHARE, 
> UVM_ADV_RANDOM, flags));
> 
>   (Note UVM_INH_SHARE == MAP_INHERIT_SHARE).

sysv shm is backed by an aobj and does not involve amap.

iirc, you need minherit to create shared-cow mappings.
(imo, the introduction of minherit was a mistake because it
merely complicated vm.)

YAMAMOTO Takashi

> 
>   I guess MAP_INHERIT_SHARE was added because adding it was easy
>   after shared amap was implemented for shared memory?
> 
> - For highly tuned, XIP'ed systems, programs should be designed to
>   avoid .data, because they're COW'ed to page cache sooner or later.