Re: [RFC PATCH] hugetlbfs: Add hugetlb_cgroup reservation limits

[Mina Almasry <almasrymina@google.com>]

On Thu, Aug 8, 2019 at 1:23 PM shuah <shuah@kernel.org> wrote:
>
> On 8/8/19 1:40 PM, Mina Almasry wrote:
> > Problem:
> > Currently tasks attempting to allocate more hugetlb memory than is available get
> > a failure at mmap/shmget time. This is thanks to Hugetlbfs Reservations [1].
> > However, if a task attempts to allocate hugetlb memory only more than its
> > hugetlb_cgroup limit allows, the kernel will allow the mmap/shmget call,
> > but will SIGBUS the task when it attempts to fault the memory in.
> >
> > We have developers interested in using hugetlb_cgroups, and they have expressed
> > dissatisfaction regarding this behavior. We'd like to improve this
> > behavior such that tasks violating the hugetlb_cgroup limits get an error on
> > mmap/shmget time, rather than getting SIGBUS'd when they try to fault
> > the excess memory in.
> >
> > The underlying problem is that today's hugetlb_cgroup accounting happens
> > at hugetlb memory *fault* time, rather than at *reservation* time.
> > Thus, enforcing the hugetlb_cgroup limit only happens at fault time, and
> > the offending task gets SIGBUS'd.
> >
> > Proposed Solution:
> > A new page counter named hugetlb.xMB.reservation_[limit|usage]_in_bytes. This
> > counter has slightly different semantics than
> > hugetlb.xMB.[limit|usage]_in_bytes:
> >
> > - While usage_in_bytes tracks all *faulted* hugetlb memory,
> > reservation_usage_in_bytes tracks all *reserved* hugetlb memory.
> >
> > - If a task attempts to reserve more memory than limit_in_bytes allows,
> > the kernel will allow it to do so. But if a task attempts to reserve
> > more memory than reservation_limit_in_bytes, the kernel will fail this
> > reservation.
> >
> > This proposal is implemented in this patch, with tests to verify
> > functionality and show the usage.
> >
> > Alternatives considered:
> > 1. A new cgroup, instead of only a new page_counter attached to
> >     the existing hugetlb_cgroup. Adding a new cgroup seemed like a lot of code
> >     duplication with hugetlb_cgroup. Keeping hugetlb related page counters under
> >     hugetlb_cgroup seemed cleaner as well.
> >
> > 2. Instead of adding a new counter, we considered adding a sysctl that modifies
> >     the behavior of hugetlb.xMB.[limit|usage]_in_bytes, to do accounting at
> >     reservation time rather than fault time. Adding a new page_counter seems
> >     better as userspace could, if it wants, choose to enforce different cgroups
> >     differently: one via limit_in_bytes, and another via
> >     reservation_limit_in_bytes. This could be very useful if you're
> >     transitioning how hugetlb memory is partitioned on your system one
> >     cgroup at a time, for example. Also, someone may find usage for both
> >     limit_in_bytes and reservation_limit_in_bytes concurrently, and this
> >     approach gives them the option to do so.
> >
> > Caveats:
> > 1. This support is implemented for cgroups-v1. I have not tried
> >     hugetlb_cgroups with cgroups v2, and AFAICT it's not supported yet.
> >     This is largely because we use cgroups-v1 for now. If required, I
> >     can add hugetlb_cgroup support to cgroups v2 in this patch or
> >     a follow up.
> > 2. Most complicated bit of this patch I believe is: where to store the
> >     pointer to the hugetlb_cgroup to uncharge at unreservation time?
> >     Normally the cgroup pointers hang off the struct page. But, with
> >     hugetlb_cgroup reservations, one task can reserve a specific page and another
> >     task may fault it in (I believe), so storing the pointer in struct
> >     page is not appropriate. Proposed approach here is to store the pointer in
> >     the resv_map. See patch for details.
> >
> > [1]: https://www.kernel.org/doc/html/latest/vm/hugetlbfs_reserv.html
> >
> > Signed-off-by: Mina Almasry <almasrymina@google.com>
> > ---
> >   include/linux/hugetlb.h                       |  10 +-
> >   include/linux/hugetlb_cgroup.h                |  19 +-
> >   mm/hugetlb.c                                  | 256 ++++++++--
> >   mm/hugetlb_cgroup.c                           | 153 +++++-
>
> Is there a reason why all these changes are in a single patch?
> I can see these split in at least 2 or 3 patches with the test
> as a separate patch.
>

Only because I was expecting feedback on the approach and alternative
approaches before an in-detail review. But, no problem; I'll break it
into smaller patches now.
> Makes it lot easier to review.
>
> thanks,
> -- Shuah