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

[Mina Almasry]

On Fri, Sep 27, 2019 at 2:59 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> 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 don't think it causes a sigbus. This is the scenario, right:

1. Make cgroup with limit X bytes.
2. Task in cgroup mmaps a file with X bytes, causing the cgroup to get charged
3. A hole of size Y is punched in the file, causing the cgroup to get
uncharged Y bytes.
4. The task faults in memory from the hole, getting charged up to Y
bytes again. But they will be still within their limits.

IIUC userspace only gets sigbus'd if the limit is lowered between
steps 3 and 4, and it's ok if it gets sigbus'd there in my opinion.

> 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