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Subject: Re: [PATCH v11 1/9] hugetlb_cgroup: Add hugetlb_cgroup reservation
 counter
To:     Mina Almasry <almasrymina@google.com>
Cc:     shuah@kernel.org, rientjes@google.com, shakeelb@google.com,
        gthelen@google.com, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-kselftest@vger.kernel.org, cgroups@vger.kernel.org
References: <20200203232248.104733-1-almasrymina@google.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <74036711-be90-a56c-3c7f-f85c30417e6e@oracle.com>
Date:   Wed, 5 Feb 2020 11:36:31 -0800
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On 2/3/20 3:22 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 9 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 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 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.rsvd.[limit|usage|max_usage]_in_bytes'. This counter has
> slightly different semantics than
> 'hugetlb.xMB.[limit|usage|max_usage]_in_bytes':
> 
> - While usage_in_bytes tracks all *faulted* hugetlb memory,
> rsvd.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 rsvd.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 rsvd.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 rsvd.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>
> Acked-by: David Rientjes <rientjes@google.com>
> 
> ---
> Changes in v11:
> - Renamed resv.* or 'reservation' or 'reserved' to rsvd.*
> - Renamed hugetlb_cgroup_get_counter() to

Thanks!
I was mostly concerned about using 'resv' in cgroup file names visible to
users.  Changing variable names is good as well.

Small nit, some lines of commit message wrap.

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
-- 
Mike Kravetz