Re: [PATCH v5 bpf-next 1/4] bpf: introduce task_vma bpf_iter

[Yonghong Song <yhs@fb.com>]

On 2/9/21 7:00 PM, Alexei Starovoitov wrote:
> On 2/9/21 2:08 PM, Song Liu wrote:
>>
>>
>>> On Feb 9, 2021, at 1:30 PM, Alexei Starovoitov 
>>> <alexei.starovoitov@gmail.com> wrote:
>>>
>>> On Mon, Feb 08, 2021 at 02:52:52PM -0800, 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@fb.com>
>>>> ---
>>>> kernel/bpf/task_iter.c | 217 ++++++++++++++++++++++++++++++++++++++++-
>>>> 1 file changed, 216 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>>>> index 175b7b42bfc46..a0d469f0f481c 100644
>>>> --- a/kernel/bpf/task_iter.c
>>>> +++ b/kernel/bpf/task_iter.c
>>>> @@ -286,9 +286,198 @@ 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;
>>>
>>> in case of contention the vma will be seen by bpf prog again?
>>> It looks like the 4 cases of overlaping vmas (after newly acquired lock)
>>> that show_smaps_rollup() is dealing with are not handled here?
>>
>> I am not sure I am following here. The logic below should avoid showing
>> the same vma again:
>>     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;
>>
>> This logic handles case 1, 2, 3 same as show_smaps_rollup(). For case 4,
>> this logic skips the changed vma (from [prev_vm_start, prev_vm_end] to
>> [prev_vm_start, prev_vm_end + something]); while show_smaps_rollup() will
>> process the new vma.  I think skipping or processing the new vma are both
>> correct, as we already processed part of it [prev_vm_start, prev_vm_end]
>> once.
> 
> Got it. Yeah, if there is a new vma that has extra range after
> prem_vm_end while prev_vm_start(s) are the same, arguably,
> bpf prog shouldn't process the same range again,
> but it will miss the upper part of the range.
> In other words there is no equivalent here to 'start'
> argument that smap_gather_stats has.
> It's fine, but lets document this subtle difference.
> 
>>>
>>>> +        } 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;
>>>
>>> what will happen if there was no contetion on the lock and no seq_stop
>>> when this line was hit and set op = find_vma; ?
>>> If I'm reading this correctly prev_vm_start/end could still
>>> belong to some previous task.
>>
>> In that case, we should be in "curr_tid != info->tid" path, no?
>>
>>> My understanding that if read buffer is big the bpf_seq_read()
>>> will keep doing while(space in buffer) {seq->op->show(), 
>>> seq->op->next();}
>>> and task_vma_seq_get_next() will iterate over all vmas of one task and
>>> will proceed into the next task, but if there was no contention and 
>>> no stop
>>> then prev_vm_end will either be still zero (so find_vma(mm, 0 - 1) 
>>> will be lucky
>>> and will go into first vma of the new task) or perf_vm_end is some 
>>> address
>>> of some previous task's vma. In this case find_vma may return wrong vma
>>> for the new task.
>>> It seems to me prev_vm_end/start should be set by this 
>>> task_vma_seq_get_next()
>>> function instead of relying on stop callback.
> 
> Yeah. I misread where the 'op' goes.
> But I think the problem still exists. Consider the loop of
> show;next;show;next;...
> Here it will be: case first_vma; case next_vma; case next_vma;
> Now it goes into new task and 'curr_tid != info->tid',
> so it does op = first_vma and info->tid = curr_tid.
> But we got unlucky and the process got suspended (with ctrl-z)
> and mmap_read_lock_killable returned eintr.
> The 'if' below will jump to finish.
> It will set info->task = NULL
> The process suppose to continue sys_read after resume.
> It will come back here to 'again:', but now it will do 'case find_vma'
> and will search for wrong prev_vm_end.

Thanks for catching this. I have discussed with Song about return value
for mmap_read_lock_killable(). We only considered ctrl-c case but
did not realize ctrl-z case :-(

Song, you can return a ERR_PTR(-EAGAIN) here. This ERR_PTR will be
available to your seq_ops->stop() function as well so you can handle
properly there too.

> 
> Maybe I'm missing something again.
> It's hard for me to follow the code.
> Could you please add diagrams like show_smaps_rollup() does and
> document what happens with all the 'op's.
> 
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