Re: [PATCH] mm/kmemleak: report RCU-tasks quiescent states during the scan

[Breno Leitao <leitao@debian.org>]

On Mon, Jul 27, 2026 at 06:04:37AM -0700, Breno Leitao wrote:
> > > The document was not a waste of time. it helped people like me to
> > > understand what the issue is, and what are the decisions we have ahead
> > > of us.
> > > 
> > > Back to what is the best decision/design, I honestly don't have an
> > > opinion, but I am happy to get more data about Meta production servers
> > > to help with the decision.
> > 
> > Looking forward to seeing what you come up with!
> 
> I found 3 different cases on Meta fleet, where rcu task stalls show up:
> 
> 1) kmemleak -> This patch solves it
> 2) KVM / kcompactd
>   * Holdout: kcompactd0 (pid 1216), state:R, nvcsw frozen at
>      716336/716336 across three reports 10 min apart (stuck ≥20 min in
>      one compaction pass)
>   * This is coming from: migrate_pages ->
>     kvm_mmu_notifier_invalidate_range_start ->
> 	   tdp_mmu_next_root -> tdp_mmu_zap_leafs
> 
> 3) Nvidia driver
>   * stuck in nv_procfs_read_lock_params
> 
> Given we can't do much about 3) and 1) is now closed, I will spend some
> time geting more data and possible a fix for 2.

Quick update about item number 2), I've investigated and found that
migrate_pages_batch() unmaps each folio before moving it, and every
unmap runs the mmu_notifier invalidate callbacks.  On KVM hosts
try_to_migrate() ends up in kvm_mmu_notifier_invalidate_range_start() ->
tdp_mmu_zap_leafs(), which is expensive. Patch at:

https://lore.kernel.org/all/20260727-kcompact-v1-1-bdfefddd6874@debian.org/