Re: [PATCH v1 5/6] memcg-v1: no need for memcg locking for MGLRU

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On Mon, Nov 04, 2024 at 01:38:34PM -0800, Andrew Morton wrote:
> On Mon, 4 Nov 2024 10:30:29 -0700 Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> 
> > On Sat, Oct 26, 2024 at 09:26:04AM -0600, Yu Zhao wrote:
> > > On Sat, Oct 26, 2024 at 12:34 AM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
> > > >
> > > > On Thu, Oct 24, 2024 at 06:23:02PM GMT, Shakeel Butt wrote:
> > > > > While updating the generation of the folios, MGLRU requires that the
> > > > > folio's memcg association remains stable. With the charge migration
> > > > > deprecated, there is no need for MGLRU to acquire locks to keep the
> > > > > folio and memcg association stable.
> > > > >
> > > > > Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > > >
> > > > Andrew, can you please apply the following fix to this patch after your
> > > > unused fixup?
> > > 
> > > Thanks!
> > 
> > syzbot caught the following:
> > 
> >   WARNING: CPU: 0 PID: 85 at mm/vmscan.c:3140 folio_update_gen+0x23d/0x250 mm/vmscan.c:3140
> >   ...
> > 
> > Andrew, can you please fix this in place?
> 
> OK, but...
> 
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -3138,7 +3138,6 @@ static int folio_update_gen(struct folio *folio, int gen)
> >  	unsigned long new_flags, old_flags = READ_ONCE(folio->flags);
> >  
> >  	VM_WARN_ON_ONCE(gen >= MAX_NR_GENS);
> > -	VM_WARN_ON_ONCE(!rcu_read_lock_held());
> >  
> >  	do {
> >  		/* lru_gen_del_folio() has isolated this page? */
> 
> it would be good to know why this assertion is considered incorrect? 
> And a link to the sysbot report?

So, this assertion is incorrect after this patch series that has removed
the charge migration and has removed mem_cgroup_trylock_pages() / 
mem_cgroup_unlock_pages() from the caller of this function
(folio_update_gen()).




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