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@xxxxxxxxxx> > Acked-by: David Rientjes <rientjes@xxxxxxxxxx> > > --- > 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@xxxxxxxxxx> -- Mike Kravetz
