Re: [PATCH 6.8.y] mm: zswap: fix shrinker NULL crash with cgroup_disable=memory

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Apr 29, 2024 at 09:02:16AM -0400, Johannes Weiner wrote:
> Christian reports a NULL deref in zswap that he bisected down to the zswap
> shrinker.  The issue also cropped up in the bug trackers of libguestfs [1]
> and the Red Hat bugzilla [2].
> 
> The problem is that when memcg is disabled with the boot time flag, the
> zswap shrinker might get called with sc->memcg == NULL.  This is okay in
> many places, like the lruvec operations.  But it crashes in
> memcg_page_state() - which is only used due to the non-node accounting of
> cgroup's the zswap memory to begin with.
> 
> Nhat spotted that the memcg can be NULL in the memcg-disabled case, and I
> was then able to reproduce the crash locally as well.
> 
> [1] https://github.com/libguestfs/libguestfs/issues/139
> [2] https://bugzilla.redhat.com/show_bug.cgi?id=2275252
> 
> Link: https://lkml.kernel.org/r/20240418124043.GC1055428@cmpxchg.org
> Link: https://lkml.kernel.org/r/20240417143324.GA1055428@cmpxchg.org
> Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Reported-by: Christian Heusel <christian@heusel.eu>
> Debugged-by: Nhat Pham <nphamcs@gmail.com>
> Suggested-by: Nhat Pham <nphamcs@gmail.com>
> Tested-by: Christian Heusel <christian@heusel.eu>
> Acked-by: Yosry Ahmed <yosryahmed@google.com>
> Cc: Chengming Zhou <chengming.zhou@linux.dev>
> Cc: Dan Streetman <ddstreet@ieee.org>
> Cc: Richard W.M. Jones <rjones@redhat.com>
> Cc: Seth Jennings <sjenning@redhat.com>
> Cc: Vitaly Wool <vitaly.wool@konsulko.com>
> Cc: <stable@vger.kernel.org>	[v6.8]
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 682886ec69d22363819a83ddddd5d66cb5c791e1)
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  mm/zswap.c | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)

Now queued up, thanks.

greg k-h