On Tue, Dec 15, 2020 at 03:36:59PM -0800, Song Liu wrote:
> +/*
> + * Key information from vm_area_struct. We need this because we cannot
> + * assume the vm_area_struct is still valid after each iteration.
> + */
> +struct __vm_area_struct {
> + __u64 start;
> + __u64 end;
> + __u64 flags;
> + __u64 pgoff;
> +};
Where it's inside .c or exposed in uapi/bpf.h it will become uapi
if it's used this way. Let's switch to arbitrary BTF-based access instead.
> +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;
> + struct task_struct *curr_task;
> + struct vm_area_struct *vma;
> + u32 curr_tid = info->tid;
> + bool new_task = false;
> +
> + /* If this function returns a non-NULL __vm_area_struct, it held
> + * a reference to the task_struct. If info->file is non-NULL, it
> + * also holds a reference to the file. If this function returns
> + * NULL, it does not hold any reference.
> + */
> +again:
> + if (info->task) {
> + curr_task = info->task;
> + } else {
> + curr_task = task_seq_get_next(ns, &curr_tid, true);
> + if (!curr_task) {
> + info->task = NULL;
> + info->tid++;
> + return NULL;
> + }
> +
> + if (curr_tid != info->tid) {
> + info->tid = curr_tid;
> + new_task = true;
> + }
> +
> + if (!curr_task->mm)
> + goto next_task;
> + info->task = curr_task;
> + }
> +
> + mmap_read_lock(curr_task->mm);
That will hurt. /proc readers do that and it causes all sorts
of production issues. We cannot take this lock.
There is no need to take it.
Switch the whole thing to probe_read style walking.
And reimplement find_vma with probe_read while omitting vmacache.
It will be short rbtree walk.
bpf prog doesn't need to see a stable struct. It will read it through
ptr_to_btf_id
which will use probe_read equivalent underneath.
