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Dave Marchevsky says: ==================== At Meta we have a profiling daemon which periodically collects information on many hosts. This collection usually involves grabbing stacks (user and kernel) using perf_event BPF progs and later symbolicating them. For user stacks we try to use BPF_F_USER_BUILD_ID and rely on remote symbolication, but BPF_F_USER_BUILD_ID doesn't always succeed. In those cases we must fall back to digging around in /proc/PID/maps to map virtual address to (binary, offset). The /proc/PID/maps digging does not occur synchronously with stack collection, so the process might already be gone, in which case it won't have /proc/PID/maps and we will fail to symbolicate. This 'exited process problem' doesn't occur very often as most of the prod services we care to profile are long-lived daemons, but there are enough usecases to warrant a workaround: a BPF program which can be optionally loaded at data collection time and essentially walks /proc/PID/maps. Currently this is done by walking the vma list: struct vm_area_struct* mmap = BPF_CORE_READ(mm, mmap); mmap_next = BPF_CORE_READ(rmap, vm_next); /* in a loop */ Since commit763ecb0350("mm: remove the vma linked list") there's no longer a vma linked list to walk. Walking the vma maple tree is not as simple as hopping struct vm_area_struct->vm_next. Luckily, commitf39af05949("mm: add VMA iterator"), another commit in that series, added struct vma_iterator and for_each_vma macro for easy vma iteration. If similar functionality was exposed to BPF programs, it would be perfect for our usecase. This series adds such functionality, specifically a BPF equivalent of for_each_vma using the open-coded iterator style. Notes: * This approach was chosen after discussion on a previous series [0] which attempted to solve the same problem by adding a BPF_F_VMA_NEXT flag to bpf_find_vma. * Unlike the task_vma bpf_iter, the open-coded iterator kfuncs here do not drop the vma read lock between iterations. See Alexei's response in [0]. * The [vsyscall] page isn't really part of task->mm's vmas, but /proc/PID/maps returns information about it anyways. The vma iter added here does not do the same. See comment on selftest in patch 3. * bpf_iter_task_vma allocates a _data struct which contains - among other things - struct vma_iterator, using BPF allocator and keeps a pointer to the bpf_iter_task_vma_data. This is done in order to prevent changes to struct ma_state - which is wrapped by struct vma_iterator - from necessitating changes to uapi struct bpf_iter_task_vma. Changelog: v6 -> v7: https://lore.kernel.org/bpf/20231010185944.3888849-1-davemarchevsky@fb.com/ Patch numbers correspond to their position in v6 Patch 2 ("selftests/bpf: Rename bpf_iter_task_vma.c to bpf_iter_task_vmas.c") * Add Andrii ack Patch 3 ("bpf: Introduce task_vma open-coded iterator kfuncs") * Add Andrii ack * Add missing __diag_ignore_all for -Wmissing-prototypes (Song) Patch 4 ("selftests/bpf: Add tests for open-coded task_vma iter") * Remove two unnecessary header includes (Andrii) * Remove extraneous !vmas_seen check (Andrii) New Patch ("bpf: Add BPF_KFUNC_{START,END}_defs macros") * After talking to Andrii, this is an attempt to clean up __diag_ignore_all spam everywhere kfuncs are defined. If nontrivial changes are needed, let's apply the other 4 and I'll respin as a standalone patch. v5 -> v6: https://lore.kernel.org/bpf/20231010175637.3405682-1-davemarchevsky@fb.com/ Patch 4 ("selftests/bpf: Add tests for open-coded task_vma iter") * Remove extraneous blank line. I did this manually to the .patch file for v5, which caused BPF CI to complain about failing to apply the series v4 -> v5: https://lore.kernel.org/bpf/20231002195341.2940874-1-davemarchevsky@fb.com/ Patch numbers correspond to their position in v4 New Patch ("selftests/bpf: Rename bpf_iter_task_vma.c to bpf_iter_task_vmas.c") * Patch 2's renaming of this selftest, and associated changes in the userspace runner, are split out into this separate commit (Andrii) Patch 2 ("bpf: Introduce task_vma open-coded iterator kfuncs") * Remove bpf_iter_task_vma kfuncs from libbpf's bpf_helpers.h, they'll be added to selftests' bpf_experimental.h in selftests patch below (Andrii) * Split bpf_iter_task_vma.c renaming into separate commit (Andrii) Patch 3 ("selftests/bpf: Add tests for open-coded task_vma iter") * Add bpf_iter_task_vma kfuncs to bpf_experimental.h (Andrii) * Remove '?' from prog SEC, open_and_load the skel in one operation (Andrii) * Ensure that fclose() always happens in test runner (Andrii) * Use global var w/ 1000 (vm_start, vm_end) structs instead of two MAP_TYPE_ARRAY's w/ 1k u64s each (Andrii) v3 -> v4: https://lore.kernel.org/bpf/20230822050558.2937659-1-davemarchevsky@fb.com/ Patch 1 ("bpf: Don't explicitly emit BTF for struct btf_iter_num") * Add Andrii ack Patch 2 ("bpf: Introduce task_vma open-coded iterator kfuncs") * Mark bpf_iter_task_vma_new args KF_RCU and remove now-unnecessary !task check (Yonghong) * Although KF_RCU is a function-level flag, in reality it only applies to the task_struct *task parameter, as the other two params are a scalar int and a specially-handled KF_ARG_PTR_TO_ITER * Remove struct bpf_iter_task_vma definition from uapi headers, define in kernel/bpf/task_iter.c instead (Andrii) Patch 3 ("selftests/bpf: Add tests for open-coded task_vma iter") * Use a local var when looping over vmas to track map idx. Update vmas_seen global after done iterating. Don't start iterating or update vmas_seen if vmas_seen global is nonzero. (Andrii) * Move getpgid() call to correct spot - above skel detach. (Andrii) v2 -> v3: https://lore.kernel.org/bpf/20230821173415.1970776-1-davemarchevsky@fb.com/ Patch 1 ("bpf: Don't explicitly emit BTF for struct btf_iter_num") * Add Yonghong ack Patch 2 ("bpf: Introduce task_vma open-coded iterator kfuncs") * UAPI bpf header and tools/ version should match * Add bpf_iter_task_vma_kern_data which bpf_iter_task_vma_kern points to, bpf_mem_alloc/free it instead of just vma_iterator. (Alexei) * Inner data ptr == NULL implies initialization failed v1 -> v2: https://lore.kernel.org/bpf/20230810183513.684836-1-davemarchevsky@fb.com/ * Patch 1 * Now removes the unnecessary BTF_TYPE_EMIT instead of changing the type (Yonghong) * Patch 2 * Don't do unnecessary BTF_TYPE_EMIT (Yonghong) * Bump task refcount to prevent ->mm reuse (Yonghong) * Keep a pointer to vma_iterator in bpf_iter_task_vma, alloc/free via BPF mem allocator (Yonghong, Stanislav) * Patch 3 [0]: https://lore.kernel.org/bpf/20230801145414.418145-1-davemarchevsky@fb.com/ ==================== Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Linux kernel
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Please read the Documentation/process/changes.rst file, as it contains the
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