Re: [PATCH v2] mm/migrate: fix softlockup in do_pages_move() with huge page arrays

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Sun, Nov 08, 2026 at 02:28:31PM +0800, shao.mingyin@zte.com.cn 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 array one page at a time.  An entry naming the
> node its page is already on is never queued, so the loop runs without a
> voluntary preemption point on a non-preemptible kernel; the existing
> cond_resched() calls in the migration code are only reached after a
> folio has been queued.  nr_pages comes straight from the syscall.
>
> A 6.6.60 x86_64 kernel with CONFIG_PREEMPT_NONE=y and CONFIG_NUMA=y
> reproduces this under QEMU TCG: 50 million entries pointing at a page
> already on the target node keep the CPU in do_pages_move() for 42.9 s and
> trip the softlockup watchdog; with this patch the same call completes in
> 44.3 s without a softlockup.
>
> Add cond_resched() so that long calls yield periodically.
> do_pages_stat() had the same problem; it was fixed by 8e934ab44389.  In
> current mainline only alpha, hexagon and classic m68k can select
> PREEMPT_NONE and they cannot enable CONFIG_NUMA, so this mainly targets
> pre-v7.0 stable kernels.
>
> Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
> Cc: stable@vger.kernel.org
> Signed-off-by: Shao Mingyin <shao.mingyin@zte.com.cn>

NAK.

This person('s script) sent 300 mails in under four weeks spanning: ata,
scsi, target, ufs, nvme/nvmet, ubifs, ublk, zloop, dm, md, mmc, virtio-blk,
mfd, gpio, pwm, spi, PCI, ASoC, ACPI, net, vfs mount, block partitions, and
now mm.

Those patches that I've looked at have actually both been wrong and both
useless.

And absolutely not to this person/script doing backports, thanks.

> ---
> Changes in v2:
>   - describe the reproduction and the code path, as requested by David
>     Hildenbrand; no code change
>   - correct the do_pages_stat() reference (fixed by 8e934ab44389)
> Link: https://lore.kernel.org/all/20261107213856462Jx8pIdgdtLvPLfOFTwV4X@zte.com.cn/
>
>  mm/migrate.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 875079464930..3122ecb85270 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2372,6 +2372,8 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>  		const void __user *p;
>  		int node;
>
> +		cond_resched();
> +
>  		err = -EFAULT;
>  		if (in_compat_syscall()) {
>  			compat_uptr_t cp;
>

--
Cheers, Lorenzo