On 2/4/21 12:49 PM, Song Liu wrote:
Introduce task_vma bpf_iter to print memory information of a process. It
can be used to print customized information similar to /proc/<pid>/maps.
Current /proc/<pid>/maps and /proc/<pid>/smaps provide information of
vma's of a process. However, these information are not flexible enough to
cover all use cases. For example, if a vma cover mixed 2MB pages and 4kB
pages (x86_64), there is no easy way to tell which address ranges are
backed by 2MB pages. task_vma solves the problem by enabling the user to
generate customize information based on the vma (and vma->vm_mm,
vma->vm_file, etc.).
To access the vma safely in the BPF program, task_vma iterator holds
target mmap_lock while calling the BPF program. If the mmap_lock is
contended, task_vma unlocks mmap_lock between iterations to unblock the
writer(s). This lock contention avoidance mechanism is similar to the one
used in show_smaps_rollup().
Signed-off-by: Song Liu <songliubraving@xxxxxx>
---
kernel/bpf/task_iter.c | 215 ++++++++++++++++++++++++++++++++++++++++-
1 file changed, 214 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index 175b7b42bfc46..31e63b6c3d718 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -286,9 +286,196 @@ static const struct seq_operations task_file_seq_ops = {
.show = task_file_seq_show,
};
+struct bpf_iter_seq_task_vma_info {
+ /* The first field must be struct bpf_iter_seq_task_common.
+ * this is assumed by {init, fini}_seq_pidns() callback functions.
+ */
+ struct bpf_iter_seq_task_common common;
+ struct task_struct *task;
+ struct vm_area_struct *vma;
+ u32 tid;
+ unsigned long prev_vm_start;
+ unsigned long prev_vm_end;
+};
+
+enum bpf_task_vma_iter_find_op {
+ task_vma_iter_first_vma, /* use mm->mmap */
+ task_vma_iter_next_vma, /* use curr_vma->vm_next */
+ task_vma_iter_find_vma, /* use find_vma() to find next vma */
+};
+
+static struct vm_area_struct *
+task_vma_seq_get_next(struct bpf_iter_seq_task_vma_info *info)
+{
+ struct pid_namespace *ns = info->common.ns;
+ enum bpf_task_vma_iter_find_op op;
+ struct vm_area_struct *curr_vma;
+ struct task_struct *curr_task;
+ u32 curr_tid = info->tid;
+
+ /* If this function returns a non-NULL vma, it holds a reference to
+ * the task_struct, and holds read lock on vma->mm->mmap_lock.
+ * If this function returns NULL, it does not hold any reference or
+ * lock.
+ */
+ if (info->task) {
+ curr_task = info->task;
+ curr_vma = info->vma;
+ /* In case of lock contention, drop mmap_lock to unblock
+ * the writer.
+ */
+ if (mmap_lock_is_contended(curr_task->mm)) {
+ info->prev_vm_start = curr_vma->vm_start;
+ info->prev_vm_end = curr_vma->vm_end;
+ op = task_vma_iter_find_vma;
+ mmap_read_unlock(curr_task->mm);
+ if (mmap_read_lock_killable(curr_task->mm))
+ goto finish;
+ } else {
+ op = task_vma_iter_next_vma;
+ }
+ } else {
+again:
+ curr_task = task_seq_get_next(ns, &curr_tid, true);
+ if (!curr_task) {
+ info->tid = curr_tid + 1;
+ goto finish;
+ }
+
+ if (curr_tid != info->tid) {
+ info->tid = curr_tid;
+ op = task_vma_iter_first_vma;
+ } else {
+ op = task_vma_iter_find_vma;
+ }
+
+ if (!curr_task->mm)
+ goto next_task;
+
+ if (mmap_read_lock_killable(curr_task->mm))
+ goto finish;
We hold a reference for curr_task here.
Going to "finish" does not release the reference.
+ }
+
+ switch (op) {
+ case task_vma_iter_first_vma:
+ curr_vma = curr_task->mm->mmap;
+ break;
+ case task_vma_iter_next_vma:
+ curr_vma = curr_vma->vm_next;
+ break;
+ case task_vma_iter_find_vma:
+ /* We dropped mmap_lock so it is necessary to use find_vma
+ * to find the next vma. This is similar to the mechanism
+ * in show_smaps_rollup().
+ */
+ curr_vma = find_vma(curr_task->mm, info->prev_vm_end - 1);
+
+ if (curr_vma && (curr_vma->vm_start == info->prev_vm_start))
+ curr_vma = curr_vma->vm_next;
+ break;
+ }
+ if (!curr_vma) {
+ mmap_read_unlock(curr_task->mm);
+ goto next_task;
+ }
+ info->task = curr_task;
+ info->vma = curr_vma;
+ return curr_vma;
+
+next_task:
+ put_task_struct(curr_task);
+ info->task = NULL;
+ curr_tid++;
+ goto again;
+
+finish:
+ info->task = NULL;
+ info->vma = NULL;
+ return NULL;
+}
+
[...]
