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.
Testing: - 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 * 2M * 32-bit 64-bit * 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 ********** - Failing tests: - elflink_rw_and_share_test("linkhuge_rw") segfaults with and without this patch series. - LD_PRELOAD=libhugetlbfs.so HUGETLB_MORECORE=yes malloc (2M: 32): FAIL Address is not hugepage - LD_PRELOAD=libhugetlbfs.so HUGETLB_RESTRICT_EXE=unknown:malloc HUGETLB_MORECORE=yes malloc (2M: 32): FAIL Address is not hugepage - LD_PRELOAD=libhugetlbfs.so HUGETLB_MORECORE=yes malloc_manysmall (2M: 32): FAIL Address is not hugepage - GLIBC_TUNABLES=glibc.malloc.tcache_count=0 LD_PRELOAD=libhugetlbfs.so HUGETLB_MORECORE=yes heapshrink (2M: 32): FAIL Heap not on hugepages - GLIBC_TUNABLES=glibc.malloc.tcache_count=0 LD_PRELOAD=libhugetlbfs.so libheapshrink.so HUGETLB_MORECORE=yes heapshrink (2M: 32): FAIL Heap not on hugepages - HUGETLB_ELFMAP=RW linkhuge_rw (2M: 32): FAIL small_data is not hugepage - HUGETLB_ELFMAP=RW HUGETLB_MINIMAL_COPY=no linkhuge_rw (2M: 32): FAIL small_data is not hugepage - alloc-instantiate-race shared (2M: 32): Bad configuration: sched_setaffinity(cpu1): Invalid argument - FAIL Child 1 killed by signal Killed - shmoverride_linked (2M: 32): FAIL shmget failed size 2097152 from line 176: Invalid argument - HUGETLB_SHM=yes shmoverride_linked (2M: 32): FAIL shmget failed size 2097152 from line 176: Invalid argument - shmoverride_linked_static (2M: 32): FAIL shmget failed size 2097152 from line 176: Invalid argument - HUGETLB_SHM=yes shmoverride_linked_static (2M: 32): FAIL shmget failed size 2097152 from line 176: Invalid argument - LD_PRELOAD=libhugetlbfs.so shmoverride_unlinked (2M: 32): FAIL shmget failed size 2097152 from line 176: Invalid argument - LD_PRELOAD=libhugetlbfs.so HUGETLB_SHM=yes shmoverride_unlinked (2M: 32): FAIL shmget failed size 2097152 from line 176: Invalid argument
Signed-off-by: Mina Almasry almasrymina@google.com
[1]: https://www.kernel.org/doc/html/latest/vm/hugetlbfs_reserv.html
Changes in v4: - Split up 'hugetlb_cgroup: add accounting for shared mappings' into 4 patches for better isolation and context on the individual changes: - hugetlb_cgroup: add accounting for shared mappings - hugetlb: disable region_add file_region coalescing - hugetlb: remove duplicated code - hugetlb: region_chg provides only cache entry - Fixed resv->adds_in_progress accounting. - Retained behavior that region_add never fails, in earlier patchsets region_add could return failure. - Fixed libhugetlbfs failure. - Minor fix to the added tests that was preventing them from running on some environments.
Changes in v3: - Addressed comments of Hillf Danton: - Added docs. - cgroup_files now uses enum. - Various readability improvements. - Addressed comments of Mike Kravetz. - region_* functions no longer coalesce file_region entries in the resv_map. - region_add() and region_chg() refactored to make them much easier to understand and remove duplicated code so this patch doesn't add too much complexity. - Refactored common functionality into helpers.
Changes in v2: - Split the patch into a 5 patch series. - Fixed patch subject.
Mina Almasry (9): hugetlb_cgroup: Add hugetlb_cgroup reservation counter hugetlb_cgroup: add interface for charge/uncharge hugetlb reservations hugetlb_cgroup: add reservation accounting for private mappings hugetlb: region_chg provides only cache entry hugetlb: remove duplicated code hugetlb: disable region_add file_region coalescing hugetlb_cgroup: add accounting for shared mappings hugetlb_cgroup: Add hugetlb_cgroup reservation tests hugetlb_cgroup: Add hugetlb_cgroup reservation docs
.../admin-guide/cgroup-v1/hugetlb.rst | 84 ++- include/linux/hugetlb.h | 24 +- include/linux/hugetlb_cgroup.h | 24 +- mm/hugetlb.c | 516 +++++++++++------- mm/hugetlb_cgroup.c | 189 +++++-- tools/testing/selftests/vm/.gitignore | 1 + tools/testing/selftests/vm/Makefile | 4 + .../selftests/vm/charge_reserved_hugetlb.sh | 440 +++++++++++++++ .../selftests/vm/write_hugetlb_memory.sh | 22 + .../testing/selftests/vm/write_to_hugetlbfs.c | 252 +++++++++ 10 files changed, 1304 insertions(+), 252 deletions(-) create mode 100755 tools/testing/selftests/vm/charge_reserved_hugetlb.sh create mode 100644 tools/testing/selftests/vm/write_hugetlb_memory.sh create mode 100644 tools/testing/selftests/vm/write_to_hugetlbfs.c
-- 2.23.0.162.g0b9fbb3734-goog