The CRIU [0] project developers are exploring potential uses of the BPF subsystem to do complicated tasks that are difficult to add support for in the kernel using existing interfaces. Even if they are implemented using procfs, or kcmp, it is difficult to make it perform well without having some kind of programmable introspection into the kernel data structures. Moreover, for procfs based state inspection, the output format once agreed upon is set in stone and hard to extend, and at the same time inefficient to consume from programs (where it is first converted from machine readable form to human readable form, only to be converted again to machine readable form). In addition to this, kcmp based file set matching algorithm performs poorly since each file in one set needs to be compared to each file in another set, to determine struct file equivalence. This set adds a io_uring file iterator (for registered files), a io_uring ubuf iterator (for registered buffers), and a epoll iterator (for registered items (files, registered using EPOLL_CTL_ADD)) to overcome these limitations. Using existing task, task_file, task_vma iterators, all of these can be combined together to significantly enhance and speed up the task dumping procedure. The two immediate use cases are io_uring checkpoint/restore support and epoll checkpoint/restore support. The first is unimplemented, and the second is being expedited using a new epoll iterator. In the future, more stages of the checkpointing sequence can be offloaded to eBPF programs to reduce process downtime, e.g. in pre-dump stage, before task is seized. The io_uring file iterator is even more important now due to the advent of descriptorless files in io_uring [1], which makes dumping a task's files a lot more harder for CRIU, since there is no visibility into these hidden descriptors that the task depends upon for operation. Similarly, the io_uring_ubuf iterator is useful in case original VMA used in registering a buffer has been destroyed. The set includes an example sample showing how these iterator(s) along with task_file iterator can be useful to restore an io_uring instance, implementing a simplified version of the code we are planning to adopt for CRIU. Patch 10 is not meant for submission, only exposition, hence explicitly marked RFC. It implements the missing features noted in [2]. Please see the individual patches for more details. [ Note (for Yonghong): I am still unusure what will be useful in show_fdinfo, fill_link_info for epoll, so that has been left out. I was reminded that io_uring now uses anon_inode_getfile_secure, which we also use in CRIU to determine source fd of ring mapping, so this should be enough to identify the io_uring fd in userspace, hence I implemented it for io_uring in v2. ] [0]: https://criu.org/Main_Page [1]: https://lwn.net/Articles/863071 [2]: https://github.com/checkpoint-restore/criu/pull/1597 Changelog: ---------- v2 -> v3: v2: https://lore.kernel.org/bpf/20211122225352.618453-1-memxor@xxxxxxxxx * Make show_fdinfo/fill_link_info functions static (Kernel Test Robot) * Minor memory leak fixes for bpf_cr * Use proper names instead of -2, -1 for denoting epoll iterator state v1 -> v2: v1: https://lore.kernel.org/bpf/20211116054237.100814-1-memxor@xxxxxxxxx * Add example showing how iterator is useful in C/R of io_uring (Alexei) * Change type of index from unsigned long to u64 (Yonghong) * Fix build error for CONFIG_IO_URING=n (Kernel Test Robot) * Move bpf_page_to_pfn out of CONFIG_IO_URING (Yonghong) * Add comment to bpf_iter_aux_info for map member (Yonghong) * show_fdinfo/fill_link_info for io_uring (Yonghong) * Fix other nits Kumar Kartikeya Dwivedi (10): io_uring: Implement eBPF iterator for registered buffers bpf: Add bpf_page_to_pfn helper io_uring: Implement eBPF iterator for registered files epoll: Implement eBPF iterator for registered items bpftool: Output io_uring iterator info selftests/bpf: Add test for io_uring BPF iterators selftests/bpf: Add test for epoll BPF iterator selftests/bpf: Test partial reads for io_uring, epoll iterators selftests/bpf: Fix btf_dump test for bpf_iter_link_info samples/bpf: Add example to checkpoint/restore io_uring fs/eventpoll.c | 201 ++++- fs/io_uring.c | 345 +++++++++ include/linux/bpf.h | 16 + include/uapi/linux/bpf.h | 18 + kernel/trace/bpf_trace.c | 19 + samples/bpf/.gitignore | 1 + samples/bpf/Makefile | 8 +- samples/bpf/bpf_cr.bpf.c | 185 +++++ samples/bpf/bpf_cr.c | 688 ++++++++++++++++++ samples/bpf/bpf_cr.h | 48 ++ samples/bpf/hbm_kern.h | 2 - scripts/bpf_doc.py | 2 + tools/bpf/bpftool/link.c | 10 + tools/include/uapi/linux/bpf.h | 18 + .../selftests/bpf/prog_tests/bpf_iter.c | 387 +++++++++- .../selftests/bpf/prog_tests/btf_dump.c | 4 +- .../selftests/bpf/progs/bpf_iter_epoll.c | 33 + .../selftests/bpf/progs/bpf_iter_io_uring.c | 50 ++ 18 files changed, 2027 insertions(+), 8 deletions(-) create mode 100644 samples/bpf/bpf_cr.bpf.c create mode 100644 samples/bpf/bpf_cr.c create mode 100644 samples/bpf/bpf_cr.h create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_epoll.c create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_io_uring.c -- 2.34.1
