Re: [PATCH 4/6] mm/damon: support flush-assisted access bit clearing for monitoring

[sashiko-bot@kernel.org]

> DAMON's access sampling clears the accessed bit of the sampled page and
> re-reads it one sampling interval later.  The clearing uses the
> non-flushing primitives, deliberately trading accuracy for low
> overhead: cached TLB translations survive the clearing, and accesses
> through such cached translations do not perform page table walks, hence
> do not set the accessed bit again.  The bit therefore re-arms at the
> TLB-refill rate rather than the actual access rate.
> 
> For most setups this is a good trade-off, since TLB pressure re-walks
> hot pages frequently anyway.  It breaks down when translations are
> long-lived, most prominently on virtualization hosts backing guest
> memory with 1 GiB hugetlb pages: covering hundreds of GiB takes only
> hundreds of TLB entries, translations essentially never get evicted,
> and secondary MMU (e.g. KVM/EPT) accessed bits cleared via
> mmu_notifier_clear_young() are rarely set again.  Measurements on a
> [ ... ]
> 
> DAMOS filter paths keep the non-flushing behavior unconditionally.
> 
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Krishna Iyer <kiyer@crusoe.ai>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260830051407.50008-1-kiyer@crusoe.ai?part=4