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@xxxxxxxxxx> > Acked-by: Hillf Danton <hdanton@xxxxxxxx> 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
