Re: [PATCH v2 19/28] mm: workingset: prevent lruvec release in workingset_refault()

[Qi Zheng]

On 12/18/25 6:30 AM, Johannes Weiner wrote:
> On Wed, Dec 17, 2025 at 03:27:43PM +0800, Qi Zheng wrote:
> > From: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >
> > In the near future, a folio will no longer pin its corresponding
> > memory cgroup. So an lruvec returned by folio_lruvec() could be
> > released without the rcu read lock or a reference to its memory
> > cgroup.
> >
> > In the current patch, the rcu read lock is employed to safeguard
> > against the release of the lruvec in workingset_refault().
> >
> > This serves as a preparatory measure for the reparenting of the
> > LRU pages.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> > ---
> >   mm/workingset.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/workingset.c b/mm/workingset.c
> > index 445fc634196d8..427ca1a5625e8 100644
> > --- a/mm/workingset.c
> > +++ b/mm/workingset.c
> > @@ -560,11 +560,12 @@ void workingset_refault(struct folio *folio, void *shadow)
> >   	 * locked to guarantee folio_memcg() stability throughout.
> >   	 */
> >   	nr = folio_nr_pages(folio);
> > +	rcu_read_lock();
> >   	lruvec = folio_lruvec(folio);
> >   	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file, nr);
> >   
> >   	if (!workingset_test_recent(shadow, file, &workingset, true))
>
> This might sleep. You have to get a reference here and unlock RCU.

Indeed, this may sleep in css_rstat_flush().

Will do the following:

get_mem_cgroup_from_folio(folio);
lruvec = mem_cgroup_lruvec(memcg, folio_pgdat(folio));

/* xxx */

mem_cgroup_put(memcg);