Re: [PATCH] mm/migrate: fix softlockup in do_pages_move() with huge page arrays

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

On 11/7/26 19:45, Zi Yan wrote:
> On Sat Nov 7, 2026 at 8:38 AM EST,  wrote:
>> From: Shao Mingyin <shao.mingyin@zte.com.cn>
>>
>> mm/migrate: fix softlockup in do_pages_move() with huge page arrays
>>
>> do_pages_move() walks the user page array one page at a time.  For each
>> entry it performs two get_user() calls, an mmap_read_lock()/unlock()
>> around a folio walk, and an optional store_status(); none of them has to
>> sleep when the arrays stay resident and the rwsem is uncontended.
>> move_pages_and_store_status() returns early while the page list is
>> empty, so an array whose entries all resolve to the same node can run
>> the whole loop without any scheduling point.  nr_pages comes straight
>> from move_pages(), so a huge array can hold a CPU past the watchdog
>> threshold on a non-preemptible kernel; with KASAN and function tracing
>> enabled, ten million entries can exceed it.
>>
>> Add cond_resched() so that long calls yield periodically.
>> do_pages_stat() has the same problem and is being fixed separately.  In
>> current mainline only alpha, hexagon and classic m68k can still select
>> PREEMPT_NONE, and they cannot enable CONFIG_NUMA, so this mainly
>> targets pre-v7.0 stable kernels.
> 
> This is probably in the same spirit as the do_pages_stat() patch[1][2].

Right, and I'd like to see an actual reproducer, because we sure have a bunch of
cond_resched() already in migration code.

-- 
Cheers,

David