Re: [PATCH v10 06/14] mm: multi-gen LRU: minimal implementation

[Chen Wandun]

在 2022/4/15 4:53, Yu Zhao 写道:
> On Thu, Apr 14, 2022 at 07:47:54PM +0800, Chen Wandun wrote:
> > On 2022/4/7 11:15, Yu Zhao wrote:
> > > +static void inc_min_seq(struct lruvec *lruvec)
> > > +{
> > > +	int type;
> > > +	struct lru_gen_struct *lrugen = &lruvec->lrugen;
> > > +
> > > +	VM_BUG_ON(!seq_is_valid(lruvec));
> > > +
> > > +	for (type = 0; type < ANON_AND_FILE; type++) {
> > > +		if (get_nr_gens(lruvec, type) != MAX_NR_GENS)
> > > +			continue;
> > I'm confused about relation between aging and LRU list operation.
> >
> > In function inc_max_seq,  both min_seq and max_seq will increase，
> > the lrugen->lists[] indexed by lru_gen_from_seq(max_seq + 1) may
> > be non-empty?
> Yes.
>
> > for example,
> > before inc_max_seq:
> > min_seq == 0, lrugen->lists[0][type][zone]
> > max_seq ==3, lrugen->lists[3][type][zone]
> >
> > after inc_max_seq:
> > min_seq ==1, lrugen->lists[1][type][zone]
> > max_seq ==4, lrugen->lists[0][type][zone]
> >
> > If lrugen->lists[0][type][zone] is not empty before inc_max_seq and it is
> > the most inactive list，however lurgen->lists[0][type][zone] will become
> > the most active list after inc_max_seq.
> Correct.
>
> > So,  in this place,
> >
> > if (get_nr_gens(lruvec, type) != MAX_NR_GENS)
> > 	continue;
> >
> > should change to
> >
> > if (get_nr_gens(lruvec, type) == MAX_NR_GENS)
> > 	continue;
> No, because max/min_seq will overlap if we do so.
>
> lrugen->lists[max_seq+1] can only be non-empty for anon LRU, for a
> couple of reasons:
> 1. We can't swap at all.
> 2. Swapping is constrained, e.g., swapfile is full.
>
> Both cases are similar to a producer (the aging) overrunning a
> consumer (the eviction). We used to handle them, but I simplified the
> code because I don't feel they are worth handling [1].

Can lrugen->lists[max_seq+1]  also be non-empty for file LRU？
such as in dont reclaim mapped file page case(isolation will fail).

If so, after aging, eviction will reclaim memory start from
lrugen->lists[min_seq+1], but some oldest file page still
remain in lrugen->lists[max_seq+1].

sort_folio can help to put misplaced pages to the right
LRU list, but in this case, it does't help, because sort_folio
only sort lrugen->lists[min_seq+1].

Thanks
Wandun
> [1] https://lore.kernel.org/r/CAOUHufbDfwgm8PgCGkhCjbhMbm=fekfjgRR56NL-j+5iUGfVuw@xxxxxxxxxxxxxx/
> .