Re: [PATCH] Expose SHM_HUGETLB in shmctl(id, IPC_STAT, ...)

[Arun Sharma <arun.sharma@google.com>]

Andrew Morton wrote:

>>>How important is this feature?
>>
>>Without this feature, an application has no way to figure out if a given 
>>segment is hugetlb or not. Applications need to know this to be able to 
>>handle alignment issues properly.
>>
>>Also, if the flag is exported via ipcs, the system administrator would 
>>have a better idea about how the hugetlb pages she configured on the 
>>system are getting used.
>>
> 
> 
> I'd suggest that any API which allows us to query the hugeness of a piece
> of memory should also work for mmap(hugetld_fd...).  IOW: this capability
> shouldn't be restricted to sysv shm areas.

The capability I was talking about was the ability to figure out where 
the configured hugetlb pages are going (vs is this a hugetlb page?).

I suspect that one can use lsof+/proc/pid/maps and look for hugetlbfs 
mount points to gather that data. But for shared memory hugepages, we 
don't have a way.

> But then again, if it was possible to write 100 lines of userspace code, we
> wouldn't need this capability at all.  I bet if the userspace guys tried a
> bit harder they'd work out a way of teaching their applications to remember
> what they did.

Why do we need shmctl(IPC_STAT) then? Applications should remember what 
they did :)

	-Arun