Re: [PATCH] mm/zsmalloc: fix NULL pointer dereference in get_next_zpdesc

[Joshua Hahn <joshua.hahnjy@gmail.com>]

I just noticed that Minchan wasn't Cc-ed in the patch. Adding him to the
conversation here.

On Mon,  9 Feb 2026 14:50:16 -0800 Joshua Hahn <joshua.hahnjy@gmail.com> wrote:

> On Mon,  9 Feb 2026 19:32:57 +0000 Michael Fara <mjfara@gmail.com> wrote:
> 
> Hello Michael,
> 
> I hope you are doing well! Thank you for the patch. I'm not entirely sure if
> the race condition that you note here is correct, and also if this is the
> right fix.
> 
> > get_next_zpdesc() calls get_zspage() which unconditionally dereferences
> > zpdesc->zspage without a NULL check. This causes a kernel oops when
> > zpdesc->zspage has been set to NULL by reset_zpdesc() during a race
> > between zspage destruction and page compaction/migration.

[...snip...]

Should we add a Fixes tag here as well?
> > Signed-off-by: Michael Fara <mjfara@gmail.com>
> > ---
> >  mm/zsmalloc.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -735,7 +735,19 @@ static struct zspage *get_zspage(struct zpdesc *zpdesc)
> > 
> >  static struct zpdesc *get_next_zpdesc(struct zpdesc *zpdesc)
> >  {
> > -	struct zspage *zspage = get_zspage(zpdesc);
> > +	struct zspage *zspage = zpdesc->zspage;
> > +
> > +	/*
> > +	 * If the backpointer is NULL, this zpdesc was already freed via
> > +	 * reset_zpdesc() by a racing async_free_zspage() while isolated
> > +	 * for compaction. See the TODO comment in zs_page_migrate().
> > +	 */
> > +	if (unlikely(!zspage)) {
> > +		WARN_ON_ONCE(1);
> > +		return NULL;
> > +	}
> > +
> > +	BUG_ON(zspage->magic != ZSPAGE_MAGIC);
> > 
> >  	if (unlikely(ZsHugePage(zspage)))
> >  		return NULL;
> > --
> > 2.39.0