Re: [PATCH] mm/page_alloc: make percpu_pagelist_high_fraction reads lock-free

[Michal Hocko <mhocko@suse.com>]

Let me add Oscar and David.

On Mon 01-12-25 09:41:12, Andrew Morton wrote:
> On Mon,  1 Dec 2025 11:30:09 +0530 Aboorva Devarajan <aboorvad@linux.ibm.com> wrote:
> 
> > When page isolation loops indefinitely during memory offline, reading
> > /proc/sys/vm/percpu_pagelist_high_fraction blocks on pcp_batch_high_lock,
> > causing hung task warnings.
> 
> That's pretty bad behavior.
> 
> I wonder if there are other problems which can be caused by this
> lengthy hold time.

pcp_batch_high_lock is not taken in any performance critical path. It is
true that memory offlining can take long when memory is not free but I
am not sure we can do much better. I guess we could check contention on
the lock and drop it to make cpu hotplug events and
sysctl_min_unmapped_ratio_sysctl_handler smoother. The question is
whether this is a practical problem hit in real life.

> It would be better to address the lengthy hold time rather that having
> to work around it in one impacted site.
> 
> > Make procfs reads lock-free since percpu_pagelist_high_fraction is a simple
> > integer with naturally atomic reads, writers still serialize via the mutex.
> > 
> > This prevents hung task warnings when reading the procfs file during
> > long-running memory offline operations.
> > 
> > ...
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -6611,11 +6611,14 @@ static int percpu_pagelist_high_fraction_sysctl_handler(const struct ctl_table *
> >  	int old_percpu_pagelist_high_fraction;
> >  	int ret;
> >  
> > +	if (!write)
> > +		return proc_dointvec_minmax(table, write, buffer, length, ppos);
> > +
> >  	mutex_lock(&pcp_batch_high_lock);
> >  	old_percpu_pagelist_high_fraction = percpu_pagelist_high_fraction;
> >  
> >  	ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
> > -	if (!write || ret < 0)
> > +	if (ret < 0)
> >  		goto out;
> >  
> >  	/* Sanity checking to avoid pcp imbalance */
> 
> That being said, I'll grab the patch and shall put a cc:stable on it,
> see what people think about this hold-time issue.

-- 
Michal Hocko
SUSE Labs