Re: [PATCH v4 15/17] mm/mglru: make folio_inc_lru_refs lruvec lockless

[sashiko-bot@kernel.org]

> From: Kairui Song <kasong@tencent.com>
> 
> The lruvec spinlock in folio_inc_lru_refs was only taken to keep
> concurrent aging from corrupting the size counters.  This is no
> longer needed: the per-generation counters are atomic, and the
> active/inactive ABI counters are updated per folio and linearized by
> the CAS on the folio flags, so concurrent aging can no longer corrupt
> them.  The same argument already allowed folio_reset_lru_refs() to
> drop the lock.
> 
> Use folio_lruvec_live_get()/folio_lruvec_live_put() to hold the RCU
> read lock around the lruvec lookup, and drop the spinlock entirely.
> 
> Without the lock, aging can advance max_seq while a promotion is in
> flight.  Generations are indexed by seq % MAX_NR_GENS, so the folio
> [ ... ]
> implies gen >= 0, and lru_gen_update_size() needs no explicit guard.
> The active/inactive ABI update keeps its gen >= 0 check because an
> off-LRU folio has no lruvec to update.
> 
> Signed-off-by: Kairui Song <kasong@tencent.com>

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

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Sashiko AI review · https://sashiko.dev/#/patchset/20261016-mglru-fg-v4-0-2106750c0bfa@tencent.com?part=15