Re: [PATCH v2] ext4: fix soft lockup while shrinking the extent status cache

[frank jpliu]

On Fri, Nov 6, 2026 at 2:30 AM Jan Kara <jack@xxxxxxx> wrote:
>
> On Wed 04-11-26 11:35:39, Jianping Liu wrote:
> > From: Jianping Liu <frankjpliu@xxxxxxxxxxx>
> >
> > __es_shrink() walks sbi->s_es_list and reclaims extents from each
> > inode. On a non-preemptible kernel that walk never gives up the CPU.
> >
> > There is a soft lockup report on our company's machine. The machine
> > with millions of active inodes, 256 CPUs, and heavy memory pressure,
> > concurrent shrinkers run too long.
> >
> > i_es_lock is dropped after es_reclaim_extents(), and s_es_lock is not
> > taken again until the next inode. When nr_to_scan remains, call
> > cond_resched_tasks_rcu_qs() before continuing the scan.
> >
> > cond_resched() does not report the RCU quiescent state needed here.
> > It is compiled out when CONFIG_PREEMPTION=y. Otherwise it schedules
> > through preempt_schedule(), and RCU records that switch as a
> > preemption. An RCU-tasks read-side section ends only at a voluntary
> > context switch, cond_resched_tasks_rcu_qs(), idle, or return to
> > userspace. The helper reports that voluntary quiescent state and still
> > lets a non-preemptible kernel reschedule.
> >
> > Changes from v1:
> > - Delete time out logic.
> > - Change "cond_resched" to "cond_resched_tasks_rcu_qs".
>
> These three lines belong below the ---. Otherwise feel free to add:

   Sorry, I had forgotten it.
   I have added  --- in the patch just now :
https://sashiko.dev/#/patchset/20261106033141.1509656-1-frankljpliu%40gmail.com

>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
>
>                                                                 Honza
>
> >
> > Link: https://patchwork.ozlabs.org/project/linux-ext4/patch/20261028061628.1209508-1-frankljpliu@xxxxxxxxx/
> > Signed-off-by: Jianping Liu <frankjpliu@xxxxxxxxxxx>
> > ---
> >  fs/ext4/extents_status.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
> > index 018b6146b9d8..62346eaf4350 100644
> > --- a/fs/ext4/extents_status.c
> > +++ b/fs/ext4/extents_status.c
> > @@ -1721,6 +1721,16 @@ static int __es_shrink(struct ext4_sb_info *sbi, int nr_to_scan,
> >
> >               if (nr_to_scan <= 0)
> >                       goto out;
> > +
> > +             /*
> > +              * Report a voluntary RCU-tasks quiescent state before continuing.
> > +              * cond_resched() is not enough: it is compiled out under
> > +              * CONFIG_PREEMPTION, and otherwise switches via preemption, which
> > +              * does not end an RCU-tasks read-side section.
> > +              * This also lets a non-preemptible kernel reschedule.
> > +              */
> > +             cond_resched_tasks_rcu_qs();
> > +
> >               spin_lock(&sbi->s_es_lock);
> >       }
> >       spin_unlock(&sbi->s_es_lock);
> > --
> > 2.43.7
> >
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR