Re: [RFC PATCH 1/2] mm: rework memcg kernel stack accounting

[Shakeel Butt]

On Wed, Aug 15, 2018 at 10:26 AM Roman Gushchin <guro@xxxxxx> wrote:
>
> On Wed, Aug 15, 2018 at 10:12:42AM -0700, Andy Lutomirski wrote:
> >
> >
> > > On Aug 15, 2018, at 9:55 AM, Roman Gushchin <guro@xxxxxx> wrote:
> > >
> > >> On Wed, Aug 15, 2018 at 12:39:23PM -0400, Johannes Weiner wrote:
> > >>> On Tue, Aug 14, 2018 at 05:36:19PM -0700, Roman Gushchin wrote:
> > >>> @@ -224,9 +224,14 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
> > >>>        return s->addr;
> > >>>    }
> > >>>
> > >>> +    /*
> > >>> +     * Allocated stacks are cached and later reused by new threads,
> > >>> +     * so memcg accounting is performed manually on assigning/releasing
> > >>> +     * stacks to tasks. Drop __GFP_ACCOUNT.
> > >>> +     */
> > >>>    stack = __vmalloc_node_range(THREAD_SIZE, THREAD_ALIGN,
> > >>>                     VMALLOC_START, VMALLOC_END,
> > >>> -                     THREADINFO_GFP,
> > >>> +                     THREADINFO_GFP & ~__GFP_ACCOUNT,
> > >>>                     PAGE_KERNEL,
> > >>>                     0, node, __builtin_return_address(0));
> > >>>
> > >>> @@ -246,12 +251,41 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
> > >>> #endif
> > >>> }
> > >>>
> > >>> +static void memcg_charge_kernel_stack(struct task_struct *tsk)
> > >>> +{
> > >>> +#ifdef CONFIG_VMAP_STACK
> > >>> +    struct vm_struct *vm = task_stack_vm_area(tsk);
> > >>> +
> > >>> +    if (vm) {
> > >>> +        int i;
> > >>> +
> > >>> +        for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
> > >>> +            memcg_kmem_charge(vm->pages[i], __GFP_NOFAIL,
> > >>> +                      compound_order(vm->pages[i]));
> > >>> +
> > >>> +        /* All stack pages belong to the same memcg. */
> > >>> +        mod_memcg_page_state(vm->pages[0], MEMCG_KERNEL_STACK_KB,
> > >>> +                     THREAD_SIZE / 1024);
> > >>> +    }
> > >>> +#endif
> > >>> +}
> > >>
> > >> Before this change, the memory limit can fail the fork, but afterwards
> > >> fork() can grow memory consumption unimpeded by the cgroup settings.
> > >>
> > >> Can we continue to use try_charge() here and fail the fork?
> > >
> > > We can, but I'm not convinced we should.
> > >
> > > Kernel stack is relatively small, and it's already allocated at this point.
> > > So IMO exceeding the memcg limit for 1-2 pages isn't worse than
> > > adding complexity and handle this case (e.g. uncharge partially
> > > charged stack). Do you have an example, when it does matter?
> >
> > What bounds it to just a few pages?  Couldnât there be lots of forks in flight that all hit this path?  Itâs unlikely, and there are surely easier DoS vectors, but still.
>
> Because any following memcg-aware allocation will fail.
> There is also the pid cgroup controlled which can be used to limit the number
> of forks.
>
> Anyway, I'm ok to handle the this case and fail fork,
> if you think it does matter.

Roman, before adding more changes do benchmark this. Maybe disabling
the stack caching for CONFIG_MEMCG is much cleaner.

Shakeel