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

[Gregory Price]

On Wed, Dec 03, 2025 at 09:27:26AM +0100, Michal Hocko wrote:
> 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@xxxxxxxxxxxxx> 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.
> 

I just today hit a scenario where offlining was blocked on migration
failures that took an exceedingly long time to offline (many minutes)
even on a relatively small block (256MB).

Now that I'm looking at the double-do-while loop in memory_hotplug.c

zone_pcp_disable(zone);  /* (pcp_batch_high_lock) */
...
do {
	do {
...
		cond_resched();
		ret = scan_movable_pages(pfn, end_pfn, &pfn);
		if (!ret) {
			/*
			 * TODO: fatal migration failures should bail
			 * out
			 */
			do_migrate_range(pfn, end_pfn);
		}
	} while (!ret);
} while (ret);
...
zone_pcp_enable(zone);  /* (pcp_batch_high_lock) */


Maybe it's time to implement the bail out?

~Gregory