Hi everyone, Another proposal from me for LSF/MM/BPF, and the last one for the time being. I'd like to discuss enabling local-storage maps (e.g. BPF_MAP_TYPE_TASK_STORAGE and BPF_MAP_TYPE_CGRP_STORAGE) to be r/o mapped directly into user space. This would allow for quick lookups of per-object state from user space, similar to how we allow it for BPF_MAP_TYPE_ARRAY, without having to do something like either of the following: - Allocating a statically sized BPF_MAP_TYPE_ARRAY which is >= the # of possible local-storage elements, which is likely wasteful in terms of memory, and which isn't easy to iterate over. - Use something like https://docs.kernel.org/bpf/bpf_iterators.html to iterate over tasks or cgroups, and collect information for each which is then dumped to user space. This would probably work, but it's not terribly performant in that it requires copying memory, trapping into the kernel, and full iteration even when it's only necessary to look up e.g. a single element. Designing and implementing this would be pretty non-trivial. We'd have to probably do a few things: 1. Write an allocator that dynamically allocates statically sized local-storage entries for local-storage maps, and populates them into pages which are mapped into user space. 2. Come up with some idr-like mechanism for mapping a local-storage object to an index into the mapping. For example, mapping a task with global pid 12345 to BPF_MAP_TYPE_TASK_STORAGE index 5, and providing ergonomic and safe ways to update these entries in the kernel and communicate them to user space. 3. Related to point 1 above, come up with some way to dynamically extend the user space mapping as more local-storage elements are added. We could potentially reserve a statically sized VA range and map all unused VA pages to the zero page, or instead possibly just leave them unmapped until they're actually needed. There are a lot of open questions, but I think it could be very useful if we can make it work. Let me know what you all think. Thanks, David
