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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 12/8/25 18:30, Aboorva Devarajan wrote:
> 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

When starting memory offlining we do a racy check whether memory 
offlining will succeed in has_unmovable_pages().

It's racy because briefly after the check, a kernel page could get 
allocated, before we isolate all the free pages.

I assume that's what happened here.

But even performing another has_unmovable_pages() check after isolating 
the free pages will not catch all cases: in particular movable pages 
that are longterm-pinned.

-- 
Cheers

David