Cc: Andrew This series is getting closer to consideration for the mm tree. Mina, Be sure to cc Andrew with next version of series.
On 10/29/19 6:36 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.
I honestly am not sure the preferred method for including the overall design in a commit message. Certainly it should be in the first patch. Perhaps, say this is patch 1 of a 9 patch series here.
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 . 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.
- 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.
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.
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.
I think that simply mentioning the use of hugetlbfs for regression testing would be sufficient here.
- Added tests passing.
- libhugetlbfs tests mostly passing, but some tests have trouble with and without this patch series. Seems environment issue rather than code:
- Overall results: ********** TEST SUMMARY
Total testcases: 84 0
Skipped: 0 0
PASS: 66 0
FAIL: 14 0
- Killed by signal: 0 0
- Bad configuration: 4 0
Expected FAIL: 0 0
Unexpected PASS: 0 0
- Test not present: 0 0
- Strange test result: 0 0
It is curious that you only ran the tests for 32 bit applications. Certainly the more common case today is 64 bit. I don't think there are any surprises for you as I also have been running hugetlbfs on this series.