Re: [PATCH v5 0/7] hugetlb_cgroup: Add hugetlb_cgroup reservation limits

[Mike Kravetz]

On 9/26/19 5:55 PM, Mina Almasry wrote:
> Provided we keep the existing controller untouched, should the new
> controller track:
> 
> 1. only reservations, or
> 2. both reservations and allocations for which no reservations exist
> (such as the MAP_NORESERVE case)?
> 
> I like the 'both' approach. Seems to me a counter like that would work
> automatically regardless of whether the application is allocating
> hugetlb memory with NORESERVE or not. NORESERVE allocations cannot cut
> into reserved hugetlb pages, correct?

Correct.  One other easy way to allocate huge pages without reserves
(that I know is used today) is via the fallocate system call.

>                                       If so, then applications that
> allocate with NORESERVE will get sigbused when they hit their limit,
> and applications that allocate without NORESERVE may get an error at
> mmap time but will always be within their limits while they access the
> mmap'd memory, correct?

Correct.  At page allocation time we can easily check to see if a reservation
exists and not charge.  For any specific page within a hugetlbfs file,
a charge would happen at mmap time or allocation time.

One exception (that I can think of) to this mmap(RESERVE) will not cause
a SIGBUS rule is in the case of hole punch.  If someone punches a hole in
a file, not only do they remove pages associated with the file but the
reservation information as well.  Therefore, a subsequent fault will be
the same as an allocation without reservation.

I 'think' the code to remove/truncate a file will work corrctly as it
is today, but I need to think about this some more.

> mmap'd memory, correct? So the 'both' counter seems like a one size
> fits all.
> 
> I think the only sticking point left is whether an added controller
> can support both cgroup-v2 and cgroup-v1. If I could get confirmation
> on that I'll provide a patchset.

Sorry, but I can not provide cgroup expertise.
-- 
Mike Kravetz