Re: [PATCH v6 1/3] workingset: refactor LRU refault to expose refault recency check

[Johannes Weiner]

On Fri, Jan 20, 2023 at 09:34:18AM -0500, Brian Foster wrote:
> On Tue, Jan 17, 2023 at 11:59:57AM -0800, Nhat Pham wrote:
> > +	int memcgid;
> > +	struct pglist_data *pgdat;
> > +	unsigned long token;
> > +
> > +	unpack_shadow(shadow, &memcgid, &pgdat, &token, workingset);
> > +	eviction_memcg = mem_cgroup_from_id(memcgid);
> > +
> > +	lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
> > +	lrugen = &lruvec->lrugen;
> > +
> > +	min_seq = READ_ONCE(lrugen->min_seq[file]);
> > +	return !((token >> LRU_REFS_WIDTH) != (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH)));
> 
> I think this might be more readable without the double negative.
> 
> Also it looks like this logic is pulled from lru_gen_refault(). Any
> reason the caller isn't refactored to use this helper, similar to how
> workingset_refault() is modified? It seems like a potential landmine to
> duplicate the logic here for cachestat purposes and somewhere else for
> actual workingset management.

The initial version was refactored. Yu explicitly requested it be
duplicated [1] to cut down on some boiler plate.

I have to agree with Brian on this one, though. The factored version
is better for maintenance than duplicating the core logic here. Even
if it ends up a bit more boiler plate - it's harder to screw that up,
and easier to catch at compile time, than the duplicates diverging.

[1] https://lore.kernel.org/lkml/CAOUHufZKTqoD2rFwrX9-eCknBmeWqP88rZ7X7A_5KHHbGBUP=A@xxxxxxxxxxxxxx/