Re: [PATCH V1] mm/slub: fix memory leak in free_to_pcs_bulk()

[Harry Yoo <harry.yoo@oracle.com>]

On Tue, Nov 11, 2025 at 05:48:35PM +0100, Tytus Rogalewski wrote:
> Do you guys still need that debug then?
> I think this is happening only when qemu vm is working.
> 
> I can get results within 1-2 days.

Hi Tythus!

Really appreciate you reporting the bug and testing it.

Now that I know what went wrong, I realize that `slab_debug=U` parameter
will hide the bug, since we disable "sheaves" feature for
debug caches.

Instead of testing with `slab_debug=U` parameter, could you please
apply this patch on top of Linux v6.18-rc5, build & install it,
and verify that the memory leak is indeed resolved on your machine?

> --
> 
> tel. 790 202 300
> 
> *Tytus Rogalewski*
> 
> Dolina Krzemowa 6A
> 
> 83-010 Jagatowo
> 
> NIP: 9570976234
> 
> 
> W dniu wt., 11 lis 2025 o 16:37 Liam R. Howlett <Liam.Howlett@oracle.com>
> napisał(a):
> 
> > * Harry Yoo <harry.yoo@oracle.com> [251111 07:55]:
> > > The commit 989b09b73978 ("slab: skip percpu sheaves for remote object
> > > freeing") introduced the remote_objects array in free_to_pcs_bulk() to
> > > skip sheaves when objects from a remote node are freed.
> > >
> > > However, the array is flushed only when:
> > >   1) the array becomes full (++remote_nr >= PCS_BATCH_MAX), or
> > >   2) slab_free_hook() returns false and size becomes zero.
> > >
> > > When neither of the conditions is met, objects in the array are leaked.
> > > This resulted in a memory leak [1], where 82 GiB of memory was allocated
> > > for the maple_node cache.
> > >
> > > Flush the array after successfully freeing objects to sheaves
> > > in the do_free: path.
> > >
> > > In the meantime, move the snippet if (!size) goto flush_remote; outside
> > > the while loop for readability. Let's say all objects in the array are
> > > from a remote node: then we acquire s->cpu_sheaves->lock and try to free
> > > an object even when size is zero. This doesn't appear to be harmful,
> > > but isn't really readable.
> > >
> > > Reported-by: Tytus Rogalewski <tytanick@gmail.com>
> > > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220765
> > > Closes:
> > https://lore.kernel.org/linux-mm/20251107094809.12e9d705b7bf4815783eb184@linux-foundation.org
> > > Closes: https://lore.kernel.org/all/aRGDTwbt2EIz2CYn@hyeyoo
> > > Fixes: 989b09b73978 ("slab: skip percpu sheaves for remote object
> > freeing")
> > > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> >
> >
> > Thanks Harry.
> >
> > Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> >
> > > ---
> > >  mm/slub.c | 8 ++++++--
> > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index f1a5373eee7b..a787687a0d59 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -6332,8 +6332,6 @@ static void free_to_pcs_bulk(struct kmem_cache *s,
> > size_t size, void **p)
> > >
> > >               if (unlikely(!slab_free_hook(s, p[i], init, false))) {
> > >                       p[i] = p[--size];
> > > -                     if (!size)
> > > -                             goto flush_remote;
> > >                       continue;
> > >               }
> > >
> > > @@ -6348,6 +6346,9 @@ static void free_to_pcs_bulk(struct kmem_cache *s,
> > size_t size, void **p)
> > >               i++;
> > >       }
> > >
> > > +     if (!size)
> > > +             goto flush_remote;
> > > +
> > >  next_batch:
> > >       if (!local_trylock(&s->cpu_sheaves->lock))
> > >               goto fallback;
> > > @@ -6402,6 +6403,9 @@ static void free_to_pcs_bulk(struct kmem_cache *s,
> > size_t size, void **p)
> > >               goto next_batch;
> > >       }
> > >
> > > +     if (remote_nr)
> > > +             goto flush_remote;
> > > +
> > >       return;
> > >
> > >  no_empty:
> > > --
> > > 2.43.0
> > >
> >

-- 
Cheers,
Harry / Hyeonggon