Re: [PATCH] mm/madvise: add cond_resched() in THP branch of madvise_cold_or_pageout_pte_range()

[Lorenzo Stoakes (ARM)]

On Mon, Oct 12, 2026 at 01:02:59PM +0200, David Hildenbrand (Arm) wrote:
> On 10/10/26 04:40, albin_yang@xxxxxxx wrote:
> > From: Wei Yang <albinwyang@xxxxxxxxxxx>
> >
> > During stress testing with stress-ng on a 256-CPU system, a softlockup
> > was observed when madvise(MADV_COLD) was called on a 31GB memory region
> > backed by transparent hugepages. The region contains 15935 THP PMD
> > entries, and the softlockup was triggered after 14485 of them had been
> > processed.
> >
> > The THP branch of madvise_cold_or_pageout_pte_range() has no scheduling
> > point. walk_pmd_range() invokes it once per 2MB PMD entry with no
> > cond_resched() anywhere in the loop. Each iteration calls
> > pmdp_invalidate() which issues a synchronous TLB shootdown (dsb ish)
> > waiting for all CPUs to acknowledge. This operation can be
> > time-consuming under heavy system load, causing the accumulated time
> > across all PMD entries to exceed the softlockup threshold (20s) and
> > panic when softlockup_panic=1. On a CONFIG_PREEMPT_NONE=y kernel, there
> > is no other opportunity to reschedule during this entire traversal.
> >
> > The regular PTE path already has an unconditional cond_resched() at the
> > function tail (introduced by commit 9c276cc65a58 ("mm: introduce
> > MADV_COLD") and commit 1a4e58cce84e ("mm: introduce MADV_PAGEOUT") when
> > MADV_COLD and MADV_PAGEOUT were first introduced) and a conditional one
> > inside the loop (commit b2f557a21bc8 ("mm/madvise: add cond_resched()
> > in madvise_cold_or_pageout_pte_range()")). The THP branch never
> > received equivalent treatment.
>
> That's quite a text to say "Do it just like the PTE case" ;)

Yeah would it be possible to cut down these giant paragraphs please?

Maybe:

	Running stress-ng on a 256-CPU system resulted in softlockups when
	madvise(MADV_COLD) was called over a 31 GiB THP-populated memory
	region with CONFIG_PREEMPT_NONE set.

	This is a result of a missing cond_resched() in the THP PMD path in
	madvise_cold_or_pageout_pte_range(), so fix the issue by doing the
	same thing the PTE path does by adding the missing cond_resched().

>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

The logic LGTM though, so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

--
Cheers, Lorenzo