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

[Andrew Morton <akpm@osdl.org>]

Arun Sharma <arun.sharma@google.com> wrote:
>
>  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?).

Well, please figure out a way which has less risk of breaking userspace.

Bear in mind that the sort of apps we're talking about here are
dubiously-written monsters with long and costly upgrade cycles and we tend
to not get any reports until many many months after we made a kernel
change.  It's very costly all round and we need to be cautious.