Re: [BUG] 4.4.x-rt - memcg: refill_stock() use get_cpu_light() has data corruption issue

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 22 Nov 2017 06:36:45 +0100
Mike Galbraith <efault@gmx.de> wrote:

> On Tue, 2017-11-21 at 22:50 -0500, Steven Rostedt wrote:
> > 
> > Does it work if you revert the patch?  
> 
> That would restore the gripe.  How about this..

Would it?

The gripe you report is:

       refill_stock()
          get_cpu_var()
          drain_stock()
             res_counter_uncharge()
                res_counter_uncharge_until()
                   spin_lock() <== boom


But commit 3e32cb2e0a1 ("mm: memcontrol: lockless page counters")
changed that code to this:

 static void drain_stock(struct memcg_stock_pcp *stock)
 {
        struct mem_cgroup *old = stock->cached;
 
        if (stock->nr_pages) {
-               unsigned long bytes = stock->nr_pages * PAGE_SIZE;
-
-               res_counter_uncharge(&old->res, bytes);
+               page_counter_uncharge(&old->memory, stock->nr_pages);
                if (do_swap_account)
-                       res_counter_uncharge(&old->memsw, bytes);
+                       page_counter_uncharge(&old->memsw, stock->nr_pages);
                stock->nr_pages = 0;
        }

Where we replaced res_counter_uncharge() which is this:

u64 res_counter_uncharge_until(struct res_counter *counter,
                               struct res_counter *top,
                               unsigned long val)
{
        unsigned long flags;
        struct res_counter *c;
        u64 ret = 0;

        local_irq_save(flags);
        for (c = counter; c != top; c = c->parent) {
                u64 r;
                spin_lock(&c->lock);
                r = res_counter_uncharge_locked(c, val);
                if (c == counter)
                        ret = r;
                spin_unlock(&c->lock);
        }
        local_irq_restore(flags);
        return ret;
}

u64 res_counter_uncharge(struct res_counter *counter, unsigned long val)
{
        return res_counter_uncharge_until(counter, NULL, val);
}

and has that spin lock, to this:

void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
{
	long new;

	new = atomic_long_sub_return(nr_pages, &counter->count);
	/* More uncharges than charges? */
	WARN_ON_ONCE(new < 0);
}

void page_counter_uncharge(struct page_counter *counter, unsigned long nr_pages)
{
	struct page_counter *c;

	for (c = counter; c; c = c->parent)
		page_counter_cancel(c, nr_pages);
}

You see. No more spin lock to gripe about. No boom in your scenario.

-- Steve