Re: [PATCH v2] ext4: fix shrinker scan budget accounting in ext4_es_scan()

[sashiko-bot@kernel.org]

> The extents_status shrinker's count_objects() callback,
> ext4_es_count(), reports the number of shrinkable extent_status
> objects via a percpu counter. do_shrink_slab() reads this count
> exactly once per invocation and derives a one-shot scan budget
> (total_scan) from it, then repeatedly calls scan_objects() in
> fixed-size batches until that budget is exhausted.
> 
> ext4_es_scan() never updates sc->nr_scanned, even though
> include/linux/shrinker.h documents that "the callee should track
> its actual progress" in that field. Because sc->nr_scanned defaults
> to sc->nr_to_scan before every call, do_shrink_slab() always
> believes a full batch was examined, regardless of what __es_shrink()
> actually did. When __es_shrink() makes no progress and returns having
> examined nothing, do_shrink_slab() has no way to tell "there was
> nothing to scan" from "a full batch was scanned and none of it was
> [ ... ]
> nr_skipped stayed at 0 throughout every run, confirming the wasted
> calls came from the stale one-shot budget racing ahead of the real
> list state, not from the existing precached/trylock skip paths.
> 
> Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>

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

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Sashiko AI review · https://sashiko.dev/#/patchset/20260929-fix-ext4-es-scan-nr-scanned-v2-1-f4e8f6f6b8b1@gmail.com?part=1