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

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

On Mon, 2025-12-01 at 09:41 -0800, 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.
> 
> It would be better to address the lengthy hold time rather that having
> to work around it in one impacted site.


Sorry for the delayed response, I spent some time recreating this issue.


I've encountered this lengthy hold time several times during memory hot-unplug, with
operations hanging indefinitely (20+ hours). It occurs intermittently, and it has 
different failure signatures, here's one example where isolation fails on a single
slab page continuously:

..
[83310.373699] page dumped because: isolation failed
[83310.373704] failed to isolate pfn 4dc68
[83310.373708] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dc68
[83310.373714] flags: 0x23ffffe00000000(node=2|zone=0|lastcpupid=0x1fffff)
[83310.373722] page_type: f5(slab)
[83310.373727] raw: 023ffffe00000000 c0000028e001fa00 5deadbeef0000100 5deadbeef0000122
[83310.373735] raw: 0000000000000000 0000000001e101e1 00000002f5000000 0000000000000000
[83310.373741] page dumped because: isolation failed
[83310.373749] failed to isolate pfn 4dc68
[83310.373753] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dc68
[83310.373760] flags: 0x23ffffe00000000(node=2|zone=0|lastcpupid=0x1fffff)
[83310.373767] page_type: f5(slab)
[83310.373770] raw: 023ffffe00000000 c0000028e001fa00 5deadbeef0000100 5deadbeef0000122
[83310.373774] raw: 0000000000000000 0000000001e101e1 00000002f5000000 0000000000000000
[83310.373778] page dumped because: isolation failed
[83310.373788] failed to isolate pfn 4dc68
[83310.373791] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dc68
[83310.373794] flags: 0x23ffffe00000000(node=2|zone=0|lastcpupid=0x1fffff)
[83310.373797] page_type: f5(slab)
[83310.373799] raw: 023ffffe00000000 c0000028e001fa00 5deadbeef0000100 5deadbeef0000122
[83310.373803] raw: 0000000000000000 0000000001e101e1 00000002f5000000 0000000000000000
[83310.373809] page dumped because: isolation failed
[83315.383370] do_migrate_range: 1098409 callbacks suppressed
[83315.383377] failed to isolate pfn 4dc68
[83315.383406] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dc68
[83315.383411] flags: 0x23ffffe00000000(node=2|zone=0|lastcpupid=0x1fffff)
[83315.383416] page_type: f5(slab)
[83315.383420] raw: 023ffffe00000000 c0000028e001fa00 5deadbeef0000100 5deadbeef0000122
[83315.383423] raw: 0000000000000000 0000000001e101e1 00000002f5000000 0000000000000000
[83315.383426] page dumped because: isolation failed
[83315.383431] failed to isolate pfn 4dc68
[83315.383433] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dc68
[83315.383442] flags: 0x23ffffe00000000(node=2|zone=0|lastcpupid=0x1fffff)
[83315.383448] page_type: f5(slab)
[83315.383454] raw: 023ffffe00000000 c0000028e001fa00 5deadbeef0000100 5deadbeef0000122
[83315.383462] raw: 0000000000000000 0000000001e101e1 00000002f5000000 0000000000000000
[83315.383470] page dumped because: isolation failed
...
...
...


Given the following statement in the documentation, should this behavior be considered
expected?

From Documentation/admin-guide/mm/memory-hotplug.rst:
"Further, memory offlining might retry for a long time (or even forever), until
aborted by the user."


There's also a TODO in the code that confirms this issue:

mm/memory_hotplug.c
/*
 * TODO: fatal migration failures should bail
 * out
 */
do_migrate_range(pfn, end_pfn);


A possible improvement would be to add a retry limit or timeout for pages that repeatedly
fail isolation, returning -EBUSY after N attempts instead of looping indefinitely for
umovable pages. This would make the behavior more predictable.


-----


In addition to the above, I've also seen test_pages_isolated() return -EBUSY at the final
isolation check for the same page-block continuously

int test_pages_isolated(unsigned long start_pfn, unsigned long end_pfn,
			enum pb_isolate_mode mode)
{
	...

	/* Check all pages are free or marked as ISOLATED */
	zone = page_zone(page);
	spin_lock_irqsave(&zone->lock, flags);
	pfn = __test_page_isolated_in_pageblock(start_pfn, end_pfn, mode); 
	spin_unlock_irqrestore(&zone->lock, flags);

	ret = pfn < end_pfn ? -EBUSY : 0; 
        ...

out:
	...
	return ret;
}

When __test_page_isolated_in_pageblock() encounters a page that isn't PageBuddy, PageHWPoison,
or PageOffline with count 0, it returns that pfn, causing -EBUSY. 


I'll work on capturing more traces for this failure scenario and follow up.

> 
> > 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.

Thanks.


Regards,
Aboorva