linux-kernel - Re: [PATCH v4 2/2] mm: avoid slub allocation while holding list

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Date:   Mon, 11 Nov 2019 10:15:20 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Christopher Lameter <cl@...ux.com>, Roman Gushchin <guro@...com>
Cc:     Yu Zhao <yuzhao@...gle.com>, Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v4 2/2] mm: avoid slub allocation while holding list_lock

+Roman Gushchin

On Mon, Nov 11, 2019 at 7:47 AM Christopher Lameter <cl@...ux.com> 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.