On Sat, Jun 20, 2020 at 1:07 PM Matt Pallissard <matt@xxxxxxxxxxxxxx> wrote:
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> On 2020-06-20T11:11:55 -0700, Yonghong Song wrote:
> >
> >
> > On 6/20/20 9:22 AM, Matt Pallissard wrote:
> > > New to bpf here.
> > >
> > > I'm trying to read values out of of mm_struct. I have code like this;
> > >
> > > unsigned long i[10] = {};
> > > struct task_struct *t;
> > > struct mm_rss_stat *rss;
> > >
> > > t = (struct task_struct *)bpf_get_current_task();
> > > BPF_CORE_READ_INTO(&rss, t, mm, rss_stat);
> > > BPF_CORE_READ_INTO(i, rss, count);
> > >
> > > However, all values in `i` appear to be 0 (i[MM_FILEPAGES], etc), as if no data gets copied. I'm about 100% confident that this is caused by a glaring oversight on my part.
> >
> > Maybe you want to check the return value of BPF_CORE_READ_INTO.
> > Underlying it is using bpf_probe_read and bpf_probe_read may fail e.g., due
> > to major fault.
>
> Doh, I should have known to check the return codes! Yes, it was failing. I knew I was overlooking something trivial.
>
I wrote exactly such piece of code a while ago. Here's part of it for
reference, I think it will be helpful:
struct task_struct *task = (struct task_struct *)bpf_get_current_task();
const struct mm_struct *mm = BPF_CORE_READ(task, mm);
if (mm) {
u64 hiwater_rss = BPF_CORE_READ(mm, hiwater_rss);
u64 file_pages = BPF_CORE_READ(mm, rss_stat.count[MM_FILEPAGES].counter);
u64 anon_pages = BPF_CORE_READ(mm, rss_stat.count[MM_ANONPAGES].counter);
u64 shmem_pages = BPF_CORE_READ(mm,
rss_stat.count[MM_SHMEMPAGES].counter);
u64 active_rss = file_pages + anon_pages + shmem_pages;
/* ... */
}
> Thanks a bunch.
>
> Matt Pallissard
