Re: [RFC PATCH v2 4/5] hugetlb_cgroup: Add accounting for shared mappings

[Mina Almasry <almasrymina@google.com>]

On Wed, Aug 14, 2019 at 9:46 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 8/13/19 4:54 PM, Mike Kravetz wrote:
> > On 8/8/19 4:13 PM, Mina Almasry wrote:
> >> For shared mappings, the pointer to the hugetlb_cgroup to uncharge lives
> >> in the resv_map entries, in file_region->reservation_counter.
> >>
> >> When a file_region entry is added to the resv_map via region_add, we
> >> also charge the appropriate hugetlb_cgroup and put the pointer to that
> >> in file_region->reservation_counter. This is slightly delicate since we
> >> need to not modify the resv_map until we know that charging the
> >> reservation has succeeded. If charging doesn't succeed, we report the
> >> error to the caller, so that the kernel fails the reservation.
> >
> > I wish we did not need to modify these region_() routines as they are
> > already difficult to understand.  However, I see no other way with the
> > desired semantics.
> >
>
> I suspect you have considered this, but what about using the return value
> from region_chg() in hugetlb_reserve_pages() to charge reservation limits?
> There is a VERY SMALL race where the value could be too large, but that
> can be checked and adjusted at region_add time as is done with normal
> accounting today.

I have not actually until now; I didn't consider doing stuff with the
resv_map while not holding onto the resv_map->lock. I guess that's the
small race you're talking about. Seems fine to me, but I'm more
worried about hanging off the vma below.

> If the question is, where would we store the information
> to uncharge?, then we can hang a structure off the vma.  This would be
> similar to what is done for private mappings.  In fact, I would suggest
> making them both use a new cgroup reserve structure hanging off the vma.
>

I actually did consider hanging off the info to uncharge off the vma,
but I didn't for a couple of reasons:

1. region_del is called from hugetlb_unreserve_pages, and I don't have
access to the vma there. Maybe there is a way to query the proper vma
I don't know about?
2. hugetlb_reserve_pages seems to be able to conduct a reservation
with a NULL *vma. Not sure what to do in that case.

Is there a way to get around these that I'm missing here?

FWIW I think tracking is better in resv_map since the reservations are
in resv_map themselves. If I do another structure, then for each
reservation there will be an entry in resv_map and an entry in the new
structure and they need to be kept in sync and I need to handle errors
for when they get out of sync.

> One issue I see is what to do if a vma is split?  The private mapping case
> 'should' handle this today, but I would not be surprised if such code is
> missing or incorrect.
>
> --
> Mike Kravetz