Re: [RFC] [RFT] Shared /dev/zero mmaping feature

[kanoj@google.engr.sgi.com (Kanoj Sarcar)]

> Why do you use this special zero_id stuff? It clutters up the whole
> code.

The zero_id stuff is required to differentiate between the struct 
shmid_kernel of a "real" shared memory segment and one that
represents a /dev/zero mapping. This is used in shm_swap_core(),
for accounting purposes, but that can be changed by adding a 
new flag to shm_swap_core. The more important use is in
shm_nopage().

> 
> If you would simply open a normal shm segment with key IPC_PRIVATE and
> directly remove it nobody can attach to it and it will be released on
> exit and everything. No special handling needed any more. BTW that's
> exectly what we do in user space to circumvent the missing MAP_ANON |
> MAP_SHARED.

Would this need to be done for each /dev/zero mapping? If so, then 
I prefer the way the code is right now, since the interference with
"real" shared memory is minimal. I was also trying to look for a way 
to prevent the zshmid_kernel hacks in shmat/shmctl (including setting
strict permissions), but couldn't come up with one ... Tell me in more
detail what you are suggesting here.

> 
> I would also prefer to be able to see the allocated segments with the
> ipc* commands.
>
I do not believe there is any good reason to expose the special shared
memory segment used as a place holder for all /dev/zero mappings to users
via ipc* commands. This special segment exists only because we 
want to reduce kernel code duplication, and all the zshmid_kernel/
zero_id checks just make sure that regular shared memory works
pretty much the way it did before. (One thing I am unhappy about
is that this special segment eats up a shm id, but that's probably
not too bad). 
 
Thanks.

Kanoj
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