[PATCH v5 1/5] PID namespace translation support

[Dmitry V. Levin]

On Tue, Aug 04, 2020 at 07:52:46PM +0200, Ákos Uzonyi wrote:
> On Mon, 3 Aug 2020 at 23:43, Dmitry V. Levin <ldv at altlinux.org> wrote:
> > On Mon, Aug 03, 2020 at 09:19:12PM +0200, Ákos Uzonyi wrote:
> > [...]
> > > +void
> > > +pidns_init(void)
> > > +{
> > > +     if (proc_data_cache)
> > > +             return;
> > > +
> > > +     pid_max = INT_MAX;
> > > +     if (read_int_from_file("/proc/sys/kernel/pid_max", &pid_max) < 0)
> > > +             debug_func_perror_msg("reading /proc/sys/kernel/pid_max");
> > > +     pid_max_size = ilog2_32(pid_max - 1) + 1;
> > > +     pid_max_size_lg = ilog2_32(pid_max_size - 1) + 1;
> > > +
> > > +     for (int i = 0; i < PT_COUNT; i++)
> > > +             ns_pid_to_proc_pid[i] = trie_create(64, ptr_sz_lg, 10, 10, 0);
> >
> > Has the numeric constant 64 been chosen because
> > sizeof(struct tcb.pid_ns) * 8 == 64?
> 
> Yes.
> For making it clear in the code, which one would be preferred?
> - Using a named constant

It's always nice to have a good name for a constant.

> - Using the expression you wrote above

Strictly speaking, that was not a valid C expression, but
	sizeof_field(struct tcb, pid_ns) * 8
is a valid one.

> - Adding a comment about it
> 
> > > +
> > > +     proc_data_cache = trie_create(pid_max_size, ptr_sz_lg, 10, 10, 0);
> > > +}
> > > +
> > > +static void
> > > +put_proc_pid(uint64_t ns, int ns_pid, enum pid_type type, int proc_pid)
> > > +{
> > > +     struct trie *b = (struct trie *) (uintptr_t) trie_get(ns_pid_to_proc_pid[type], ns);
> > > +     if (!b) {
> > > +             b = trie_create(pid_max_size, pid_max_size_lg, 10, 10, 0);
> > > +             trie_set(ns_pid_to_proc_pid[type], ns, (uint64_t) (uintptr_t) b);
> > > +     }
> > > +     trie_set(b, ns_pid, proc_pid);
> > > +}
> >
> > In all three cases trie_create is invoked with node_key_bits
> > and data_block_key_bits arguments equal to 10.
> >
> > Could you shed some light on the choice of this numeric constant, please?
> 
> Unfortunately, I have no strong reason why 10 was chosen. It's just a
> number that seemed to be ok. But thinking a bit more about it, I
> realized maybe a smaller number would be better. 10 means node size is
> 8KB (with 8 byte pointers), which seems to be a lot of memory usage,

Should the number depend on the size of pointer then?
Or should it depend on pid_max?

> as PIDs are sparse.

What do you mean by that?

> Maybe for example 4 could be a better choice, so
> nodes are small, and lookup times are still not that bad. But I'm
> still not sure what explanation could I write to the chosen constant.

Even if it just feels right, try to explain in the comment why it feels right.


-- 
ldv