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

Harry Yoo
2025-11-13T00:43:39Z

On Wed, Nov 12, 2025 at 03:47:52PM +0100, Tytus Rogalewski wrote:
> We wont make it until next week.
> Maybe you guys can compile newest r5 kernel with that patch ?
> We are using https://prebuiltkernels.com/
> ourselves. We can do that next week.

I built it and uploaded it to my personal server:
http://download.kerneltesting.org/linux-6.18.0-rc5-fix.zip

But if you prefer to test images from prebuiltkernels.com, I think it's
fine to wait for a week and test 6.18.0-rc6 - I guess this will land -rc6
anyway.

> This week is full of emergencies lol

Haha I see, I can imagine what'll happen when you test latest kernels...

> If you can provide me two debs like prebuild kernels i could deploy it and
> leave for testing for 1-2 days.

Thanks a lot!

> --
>
> tel. 790 202 300
>
> *Tytus Rogalewski*
>
> Dolina Krzemowa 6A
>
> 83-010 Jagatowo
>
> NIP: 9570976234
>
>
> wt., 11 lis 2025 o 19:29 Harry Yoo <harry.yoo@oracle.com> napisał(a):
>
> > 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
> >

--
Cheers,
Harry / Hyeonggon