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

[shuah <shuah@kernel.org>]

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.

Makes it lot easier to review.

thanks,
-- Shuah