From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Song Liu <songliubraving@fb.com>
Cc: "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"ast@kernel.org" <ast@kernel.org>,
"daniel@iogearbox.net" <daniel@iogearbox.net>,
"andrii@kernel.org" <andrii@kernel.org>,
"john.fastabend@gmail.com" <john.fastabend@gmail.com>,
"kpsingh@chromium.org" <kpsingh@chromium.org>,
Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf_iter
Date: Thu, 17 Dec 2020 18:34:44 -0800 [thread overview]
Message-ID: <20201218023444.i6hmdi3bp5vgxou2@ast-mbp> (raw)
In-Reply-To: <C4D9D25A-C3DD-4081-9EAD-B7A5B6B74F45@fb.com>
On Thu, Dec 17, 2020 at 10:08:31PM +0000, Song Liu wrote:
>
>
> > On Dec 17, 2020, at 11:03 AM, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> >
> > 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.
>
> rw_semaphore is designed to avoid write starvation, so read_lock should not cause
> problem unless the lock was taken for extended period. [1] was a recent fix that
> avoids /proc issue by releasing mmap_lock between iterations. We are using similar
> mechanism here. BTW: I changed this to mmap_read_lock_killable() in the next version.
>
> On the other hand, we need a valid vm_file pointer for bpf_d_path. So walking the
ahh. I missed that. Makes sense.
vm_file needs to be accurate, but vm_area_struct should be accessed as ptr_to_btf_id.
> rbtree without taking any lock would not work. We can avoid taking the lock when
> some SPF like mechanism merged (hopefully soonish).
>
> Did I miss anything?
>
> We can improve bpf_iter with some mechanism to specify which task to iterate, so
> that we don't have to iterate through all tasks when the user only want to inspect
> vmas in one task.
yes. let's figure out how to make it parametrizable.
map_iter runs only for given map_fd.
Maybe vma_iter should run only for given pidfd?
I think all_task_all_vmas iter is nice to have, but we don't really need it?
> Thanks,
> Song
>
> [1] ff9f47f6f00c ("mm: proc: smaps_rollup: do not stall write attempts on mmap_lock")
Thanks for this link. With "if (mmap_lock_is_contended())" check it should work indeed.
next prev parent reply other threads:[~2020-12-18 2:35 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-15 23:36 [PATCH v2 bpf-next 0/4] introduce bpf_iter for task_vma Song Liu
2020-12-15 23:36 ` [PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf_iter Song Liu
2020-12-16 17:36 ` Yonghong Song
2020-12-16 19:41 ` Song Liu
2020-12-17 0:34 ` Andrii Nakryiko
2020-12-17 1:51 ` Song Liu
2020-12-17 19:03 ` Alexei Starovoitov
2020-12-17 22:08 ` Song Liu
2020-12-18 2:34 ` Alexei Starovoitov [this message]
2020-12-18 3:15 ` Yonghong Song
2020-12-18 4:33 ` Song Liu
2020-12-18 5:23 ` Alexei Starovoitov
2020-12-18 16:38 ` Yonghong Song
2020-12-18 17:23 ` Song Liu
2021-01-05 1:46 ` Alexei Starovoitov
2021-01-05 5:47 ` Song Liu
2021-01-05 16:27 ` Alexei Starovoitov
2021-01-05 17:10 ` Song Liu
2021-01-05 17:27 ` Alexei Starovoitov
2021-01-05 19:38 ` Song Liu
2021-01-05 19:46 ` Alexei Starovoitov
2021-01-05 19:51 ` Song Liu
2020-12-15 23:37 ` [PATCH v2 bpf-next 2/4] bpf: allow bpf_d_path in sleepable bpf_iter program Song Liu
2020-12-16 17:41 ` Yonghong Song
2020-12-16 18:15 ` KP Singh
2020-12-16 18:31 ` KP Singh
2021-01-25 12:52 ` Jiri Olsa
2020-12-15 23:37 ` [PATCH v2 bpf-next 3/4] libbpf: introduce section "iter.s/" for " Song Liu
2020-12-16 17:42 ` Yonghong Song
2020-12-16 18:00 ` KP Singh
2020-12-15 23:37 ` [PATCH v2 bpf-next 4/4] selftests/bpf: add test for bpf_iter_task_vma Song Liu
2020-12-16 18:18 ` Yonghong Song
2020-12-16 23:23 ` Song Liu
2020-12-16 17:00 ` [PATCH v2 bpf-next 0/4] introduce bpf_iter for task_vma Yonghong Song
2020-12-16 17:35 ` Song Liu
2020-12-16 18:31 ` Yonghong Song
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