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SubjectRe: [PATCH v9 1/8] hugetlb_cgroup: Add hugetlb_cgroup reservation counter
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On 12/17/19 3:16 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 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 and
> hugetlb memory faulted in without a prior reservation.

To me, this implies that 'faults without reservations' could cause
reservation usage to exceed reservation limit? Or, does the faulting
process get a SIGBUS because of the reservation limit even though it
is not using reservations?

We shall see in subsequent patches.

>
> - 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. We also added cgroup-v2 support to
> hugetlb_cgroup so that the new use cases can be extended to v2.

As previously discussed, cgroup-v2 support for hugetlb_cgroup will exist
before this patch series.

>
> 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>
> Acked-by: Hillf Danton <hdanton@sina.com>

I think the ACK by Hillf happened some time back. You may want to check
to see if it still applies.

>
> ---
> include/linux/hugetlb.h | 4 +-
> mm/hugetlb_cgroup.c | 116 +++++++++++++++++++++++++++++++++++-----
> 2 files changed, 106 insertions(+), 14 deletions(-)

Only one minor nit in the code.

You made this cleanup,
@@ -472,7 +519,7 @@ static void __init __hugetlb_cgroup_file_dfl_init(int idx)
struct hstate *h = &hstates[idx];

/* format the size */
- mem_fmt(buf, 32, huge_page_size(h));
+ mem_fmt(buf, sizeof(buf), huge_page_size(h));

/* Add the limit file */
cft = &h->cgroup_files_dfl[0];

But did not make the same cleanup in __hugetlb_cgroup_file_legacy_init()
--
Mike Kravetz

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