On Tue, Jan 11, 2022 at 01:46:24PM +0530, Aneesh Kumar K.V wrote:
> Yu Zhao <yuzhao@xxxxxxxxxx> 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);
}
