Re: [PATCH] [0/6] HUGETLB memory commitment

[Martin J. Bligh]

> As I understood this originally, the suggestion was to reserve hugetlb 
> pages at mmap() or shm_get() time so that the user would get an -ENOMEM 
> at that time if there aren't enough hugetlb pages to (eventually) satisfy 
> the request, as per the notion that we shouldn't modify the user API due 
> to going with allocate on fault instead of hugetlb_prefault().

Yup, but there were two parts to it:

1. Stop hugepages using the existing overcommit pool for small pages, 
which breaks small page allocations by prematurely the pool.
2. Give hugepages their own over-commit pool, instead of prefaulting.

Personally I think we need both (as you seem to), but (1) is probably
more urgent.

> Since the reservation belongs to the mapped object (file or segment), 
> I've been storing the current file/segments's reservation in the file 
> system dependent part of the inode.  That way, it is easily accessible 
> when the hugetlbfs file or SysV segment is removed and we can reduce 
> the total number of reserved pages by that file's reservation at that 
> time.  This also allows us to handle the reservation in the absence 
> of a vma, as per Andy'c comment below.

Do we need to store it there, or is one central pool number sufficient?
I would have thought it was ...

> Admittedly this doesn't alow one to request that hugetlbpages be 
> overcommitted, or to handle problems caused to the "normal" page 
> overcommit code due to the presence of hugepages.  But we figure that 
> anyone that is actually using hugetlb pages is likely to take over 
> almost all of main memory anyway in a single job, so overcommit 
> doesn't make much sense to us.

Seeing as you can't swap them, overcommitting makes no sense to me
either ;-)

M.

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