On Mon, Mar 28, 2022 at 11:16:15PM IST, Hao Luo wrote: > On Mon, Mar 28, 2022 at 10:39 AM Hao Luo <haoluo@xxxxxxxxxx> wrote: > > > > Hi Yonghong, > > > > On Fri, Mar 25, 2022 at 12:16 PM Yonghong Song <yhs@xxxxxx> wrote: > > > > > > On 3/24/22 4:41 PM, Hao Luo wrote: > > > > Some map types support mmap operation, which allows userspace to > > > > communicate with BPF programs directly. Currently only arraymap > > > > and ringbuf have mmap implemented. > > > > > > > > However, in some use cases, when multiple program instances can > > > > run concurrently, global mmapable memory can cause race. In that > > > > case, userspace needs to provide necessary synchronizations to > > > > coordinate the usage of mapped global data. This can be a source > > > > of bottleneck. > > > > > > I can see your use case here. Each calling process can get the > > > corresponding bpf program task local storage data through > > > mmap interface. As you mentioned, there is a tradeoff > > > between more memory vs. non-global synchronization. > > > > > > I am thinking that another bpf_iter approach can retrieve > > > the similar result. We could implement a bpf_iter > > > for task local storage map, optionally it can provide > > > a tid to retrieve the data for that particular tid. > > > This way, user space needs an explicit syscall, but > > > does not need to allocate more memory than necessary. > > > > > > WDYT? > > > > > > > Thanks for the suggestion. I have two thoughts about bpf_iter + tid and mmap: > > > > - mmap prevents the calling task from reading other task's value. > > Using bpf_iter, one can pass other task's tid to get their values. I > > assume there are two potential ways of passing tid to bpf_iter: one is > > to use global data in bpf prog, the other is adding tid parameterized > > iter_link. For the first, it's not easy for unpriv tasks to use. For > > the second, we need to create one iter_link object for each interested > > tid. It may not be easy to use either. > > > > - Regarding adding an explicit syscall. I thought about adding > > write/read syscalls for task local storage maps, just like reading > > values from iter_link. Writing or reading task local storage map > > updates/reads the current task's value. I think this could achieve the > > same effect as mmap. > > > > Actually, my use case of using mmap on task local storage is to allow > userspace to pass FDs into bpf prog. Some of the helpers I want to add > need to take an FD as parameter and the bpf progs can run > concurrently, thus using global data is racy. Mmapable task local > storage is the best solution I can find for this purpose. > > Song also mentioned to me offline, that mmapable task local storage > may be useful for his use case. > > I am actually open to other proposals. > You could also use a syscall prog, and use bpf_prog_test_run to update local storage for current. Data can be passed for that specific prog invocation using ctx. You might have to enable bpf_task_storage helpers in it though, since they are not allowed to be called right now. > > > > > > > > It would be great to have a mmapable local storage in that case. > > > > This patch adds that. > > > > > > > > Mmap isn't BPF syscall, so unpriv users can also use it to > > > > interact with maps. > > > > > > > > Currently the only way of allocating mmapable map area is using > > > > vmalloc() and it's only used at map allocation time. Vmalloc() > > > > may sleep, therefore it's not suitable for maps that may allocate > > > > memory in an atomic context such as local storage. Local storage > > > > uses kmalloc() with GFP_ATOMIC, which doesn't sleep. This patch > > > > uses kmalloc() with GFP_ATOMIC as well for mmapable map area. > > > > > > > > Allocating mmapable memory has requirment on page alignment. So we > > > > have to deliberately allocate more memory than necessary to obtain > > > > an address that has sdata->data aligned at page boundary. The > > > > calculations for mmapable allocation size, and the actual > > > > allocation/deallocation are packaged in three functions: > > > > > > > > - bpf_map_mmapable_alloc_size() > > > > - bpf_map_mmapable_kzalloc() > > > > - bpf_map_mmapable_kfree() > > > > > > > > BPF local storage uses them to provide generic mmap API: > > > > > > > > - bpf_local_storage_mmap() > > > > > > > > And task local storage adds the mmap callback: > > > > > > > > - task_storage_map_mmap() > > > > > > > > When application calls mmap on a task local storage, it gets its > > > > own local storage. > > > > > > > > Overall, mmapable local storage trades off memory with flexibility > > > > and efficiency. It brings memory fragmentation but can make programs > > > > stateless. Therefore useful in some cases. > > > > > > > > Hao Luo (2): > > > > bpf: Mmapable local storage. > > > > selftests/bpf: Test mmapable task local storage. > > > > > > > > include/linux/bpf.h | 4 + > > > > include/linux/bpf_local_storage.h | 5 +- > > > > kernel/bpf/bpf_local_storage.c | 73 +++++++++++++++++-- > > > > kernel/bpf/bpf_task_storage.c | 40 ++++++++++ > > > > kernel/bpf/syscall.c | 67 +++++++++++++++++ > > > > .../bpf/prog_tests/task_local_storage.c | 38 ++++++++++ > > > > .../bpf/progs/task_local_storage_mmapable.c | 38 ++++++++++ > > > > 7 files changed, 257 insertions(+), 8 deletions(-) > > > > create mode 100644 tools/testing/selftests/bpf/progs/task_local_storage_mmapable.c > > > > -- Kartikeya
