Re: [PATCH v4 2/2] mm: avoid slub allocation while holding list_lock

[Shakeel Butt]

+Roman Gushchin

On Mon, Nov 11, 2019 at 7:47 AM Christopher Lameter <cl@xxxxxxxxx> wrote:
>
> On Sun, 10 Nov 2019, Yu Zhao wrote:
>
> > On Sat, Nov 09, 2019 at 11:16:28PM +0000, Christopher Lameter wrote:
> > > On Sat, 9 Nov 2019, Yu Zhao wrote:
> > >
> > > > >         struct page *page, *h;
> > > > > +       unsigned long *map = bitmap_alloc(oo_objects(s->max), GFP_KERNEL);
> > > > > +
> > > > > +       if (!map)
> > > > > +               return;
> > > >
> > > > What would happen if we are trying to allocate from the slab that is
> > > > being shut down? And shouldn't the allocation be conditional (i.e.,
> > > > only when CONFIG_SLUB_DEBUG=y)?
> > >
> > > Kmalloc slabs are never shut down.
> >
> > Maybe I'm not thinking straight -- isn't it what caused the deadlock in
> > the first place?
>
> Well if kmalloc allocations become a problem then we have numerous
> issues all over the kernel to fix.
>
> > Kmalloc slabs can be shut down when memcg is on.
>
> Kmalloc needs to work even during shutdown of a memcg.
>
> Maybe we need to fix memcg to not allocate from the current memcg during
> shutdown?
>
>

Roman recently added reparenting of memcg kmem caches on memcg offline
and can comment in more detail but we don't shutdown a kmem cache
until all the in-fly memcg allocations are resolved. Also the
allocation here does not look like a __GFP_ACCOUNT allocation.