Re: [PATCH v10 1/8] hugetlb_cgroup: Add hugetlb_cgroup reservation counter

Mike Kravetz
2020-01-16T22:44:59Z

On 1/14/20 5:26 PM, Mina Almasry wrote:
> These counters will track hugetlb reservations rather than hugetlb
> memory faulted in. This patch only adds the counter, following patches
> add the charging and uncharging of the counter.
>
> This is patch 1 of an 8 patch series.
>
> Problem:
> Currently tasks attempting to reserve 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 reserve hugetlb memory only more than its

*reword*
However, if a task attempts to reserve more hugetlb memory 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.

*reword*
but will SIGBUS the task when it attempts to fault in the excess memory.

>
> We have users hitting their hugetlb_cgroup limits and thus we've been
> looking at this failure mode. We'd like to improve this behavior such that users
> 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. This
> gives the user an opportunity to fallback more gracefully to
> non-hugetlbfs memory for example.
>
> 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|max_usage]_in_bytes'. This counter has
> slightly different semantics than

You changed the name to 'hugetlb.xMB.resv_[limit|usage|max_usage]_in_bytes'
in the code, but left this description.

Also, David suggested 'rsvd' as the abbreviation to use here. I would also
prefer that name to be consistent with other hugetlb interfaces.

> 'hugetlb.xMB.[limit|usage|max_usage]_in_bytes':
>
> - While usage_in_bytes tracks all *faulted* hugetlb memory,
> reservation_usage_in_bytes tracks all *reserved* hugetlb memory and
> hugetlb memory faulted in without a prior reservation.
>
> - 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 series, 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.
>
> Testing:
> - Added tests passing.
> - Used libhugetlbfs for regression testing.
>
> [1]: https://www.kernel.org/doc/html/latest/vm/hugetlbfs_reserv.html
>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
>
> ---
> Changes in v10:
> - Renamed reservation_* to resv.*
>
> ---
> include/linux/hugetlb.h | 4 +-
> mm/hugetlb_cgroup.c | 115 +++++++++++++++++++++++++++++++++++-----
> 2 files changed, 104 insertions(+), 15 deletions(-)

The code looks fine to me.

With the commit message and naming updates, I will add a Reviewed-by:

Please do wait a few/several days before sending a revised edition to
make sure we get all feedback. I really would like to get comments from
people more familiar with cgroups.

--
Mike Kravetz