Date:   Fri, 12 Feb 2021 01:41:39 +0000
From:   Song Liu <songliubraving@...com>
To:     Yonghong Song <yhs@...com>
CC:     Alexei Starovoitov <ast@...com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v5 bpf-next 1/4] bpf: introduce task_vma bpf_iter



> On Feb 10, 2021, at 3:02 PM, Yonghong Song <yhs@...com> wrote:
> 
> 
> 
> 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@...il.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@...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 :-(

Actually, we don't need to handle the ctrl-z case. Ctrl-z (or kill -STOP) 
will not cause mmap_read_lock_killable() to return -EINTR. I also confirmed 
this in the experiments. Something like the following will occasionally 
trigger mmap_read_lock_killable() to return -EINTR,

  cat /sys/fs/bpf/task_vma & sleep 0.0001 ; kill $!

while the following (using kill -STOP) will not:

  cat /sys/fs/bpf/task_vma & sleep 0.0001 ; kill -STOP $!

Thanks,
Song

> 
> 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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