Subject: Re: [PATCH v6 4/9] mm: multigenerational lru: groundwork

From: Yu Zhao <[email-masked]>
Date: Tue, 11 Jan 2022 19:16:15 -0700

On Tue, Jan 11, 2022 at 01:46:24PM +0530, Aneesh Kumar K.V wrote:
> Yu Zhao <[email-masked]> writes:
> 
> .....
> 
>  +
> > +/*
> > + * Evictable pages are divided into multiple generations. The youngest and the
> > + * oldest generation numbers, max_seq and min_seq, are monotonically increasing.
> > + * They form a sliding window of a variable size [MIN_NR_GENS, MAX_NR_GENS]. An
> > + * offset within MAX_NR_GENS, gen, indexes the lru list of the corresponding
> > + * generation. The gen counter in folio->flags stores gen+1 while a page is on
> > + * lrugen->lists[]. Otherwise, it stores 0.
> > + *
> > + * A page is added to the youngest generation on faulting. The aging needs to
> > + * check the accessed bit at least twice before handing this page over to the
> > + * eviction. The first check takes care of the accessed bit set on the initial
> > + * fault; the second check makes sure this page hasn't been used since then.
> > + * This process, AKA second chance, requires a minimum of two generations,
> > + * hence MIN_NR_GENS. And to be compatible with the active/inactive lru, these
> > + * two generations are mapped to the active; the rest of generations, if they
> > + * exist, are mapped to the inactive. PG_active is always cleared while a page
> > + * is on lrugen->lists[] so that demotion, which happens consequently when the
> > + * aging creates a new generation, needs not to worry about it.
> > + */
> 
> Where do we clear PG_active in the code? Is this the reason we endup
> with

We clear PG_active when we add a page (folio) to MGLRU lists:
  include/linux/mm_inline.h
    lru_gen_add_folio()
	do {
		new_flags = old_flags = READ_ONCE(folio->flags);

		...

		new_flags &= ~(LRU_GEN_MASK | BIT(PG_active));
                                                  ^^^^^^^^^
		...

	} while (cmpxchg(&folio->flags, old_flags, new_flags) != old_flags);

We also set it when we isolate a page (for page migration):
  include/linux/mm_inline.h
    lru_gen_del_folio()
	do {
		new_flags = old_flags = READ_ONCE(folio->flags);

		...

		else if (lru_gen_is_active(lruvec, gen))
			new_flags |= BIT(PG_active);
                                         ^^^^^^^^^
	} while (cmpxchg(&folio->flags, old_flags, new_flags) != old_flags);

> 
>   void deactivate_page(struct page *page)
>   {
>  -	if (PageLRU(page) && PageActive(page) && !PageUnevictable(page)) {
>  +	if (PageLRU(page) && !PageUnevictable(page) && (PageActive(page) || lru_gen_enabled())) {

That's correct.

> > +#define MIN_NR_GENS		2U
> > +#define MAX_NR_GENS		((unsigned int)CONFIG_NR_LRU_GENS)
> > +
> > +struct lru_gen_struct {
> > +	/* the aging increments the youngest generation number */
> > +	unsigned long max_seq;
> > +	/* the eviction increments the oldest generation numbers */
> > +	unsigned long min_seq[ANON_AND_FILE];
> > +	/* the birth time of each generation in jiffies */
> > +	unsigned long timestamps[MAX_NR_GENS];
> > +	/* the multigenerational lru lists */
> > +	struct list_head lists[MAX_NR_GENS][ANON_AND_FILE][MAX_NR_ZONES];
> > +	/* the sizes of the above lists */
> > +	unsigned long nr_pages[MAX_NR_GENS][ANON_AND_FILE][MAX_NR_ZONES];
> > +	/* whether the multigenerational lru is enabled */
> > +	bool enabled;
> > +};
> > +
> 
> ....
> 
> >  static void __meminit zone_init_internals(struct zone *zone, enum zone_type idx, int nid,
> > diff --git a/mm/swap.c b/mm/swap.c
> > index e8c9dc6d0377..d7dde3b7d4b5 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -462,6 +462,11 @@ void folio_add_lru(struct folio *folio)
> >  	VM_BUG_ON_FOLIO(folio_test_active(folio) && folio_test_unevictable(folio), folio);
> >  	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
> >  
> > +	/* see the comment in lru_gen_add_folio() */
> > +	if (lru_gen_enabled() && !folio_test_unevictable(folio) &&
> > +	    task_in_lru_fault() && !(current->flags & PF_MEMALLOC))
> > +		folio_set_active(folio);
> > +
> 
> 
> Can you explain this better? What is the significance of marking the
> folio active here. Do we need to differentiate parallel page faults (across
> different vmas) w.r.t task_in_lru_fault()?

All pages faulted in need to be added to the youngest generation. But
without PG_active, lru_gen_add_folio() doesn't know whether a page was
faulted in, or something else, e.g., page cache readahead. This is
because pages aren't immediately sent to lru_gen_add_folio(). They are
batched by lru_pvecs:

/**
 * folio_add_lru - Add a folio to an LRU list.
 * @folio: The folio to be added to the LRU.
 *
 * Queue the folio for addition to the LRU. The decision on whether
 * to add the page to the [in]active [file|anon] list is deferred until the
 * pagevec is drained. This gives a chance for the caller of folio_add_lru()
 * have the folio added to the active list using folio_mark_accessed().
 */
void folio_add_lru(struct folio *folio)
{
	struct pagevec *pvec;

	VM_BUG_ON_FOLIO(folio_test_active(folio) && folio_test_unevictable(folio), folio);
	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);

	/* see the comment in lru_gen_add_folio() */
	if (lru_gen_enabled() && !folio_test_unevictable(folio) &&
	    lru_gen_in_pgfault() && !(current->flags & PF_MEMALLOC))
		folio_set_active(folio);

	folio_get(folio);
	local_lock(&lru_pvecs.lock);
	pvec = this_cpu_ptr(&lru_pvecs.lru_add);
	if (pagevec_add_and_need_flush(pvec, &folio->page))
		__pagevec_lru_add(pvec);
	local_unlock(&lru_pvecs.lock);
}

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