Re: [PATCH] mm/lruvec: preemptively free dead folios during lru_add drain

[JP Kobryn (Meta)]

On 4/23/26 11:46 AM, Shakeel Butt wrote:
> On Thu, Apr 23, 2026 at 09:43:07AM -0700, JP Kobryn (Meta) wrote:
> > Of all observable lruvec lock contention in our fleet, we find that ~24%
> > occurs when dead folios are present in lru_add batches at drain time.
>
> So, when they were added to the percpu lru cache, they were alive but during
> their stay in lru cache, they were freed (last non-lrucache ref dropped) or
> somehow we are adding folio where the caller drops the reference just after
> adding to percpu lru cache e.g. folio_putback_lru() ?

Both scenarios can occur. Whether all callers put the folio while it is
on the per-cpu batch or putback drops ref from 2 to 1, the batch ref
is what remains.

[...]
> Overall the code looks good but I do wonder if we can add something similar to
> folio_add_lru() and if that would be enough.

folio_add_lru() is how it gets onto the batch. But it's still alive at
that point - at least one caller ref.