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

[Yu Zhao <yuzhao@google.com>]

On Fri, Apr 15, 2022 at 10:23:18AM +0800, Chen Wandun wrote:
> 
> 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？

On reclaim path, no. But it can be forced to do so via debugfs.

> such as in dont reclaim mapped file page case(isolation will fail).

You mean may_unmap=false? Pages stays in the same generation if
isolation fails. So lrugen->lists[min_seq] won't be empty in this
case.

> 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].

On reclaim path, inc_max_seq() is only called when need_aging=true,
and this guarantees max_seq-min_seq[LRU_GEN_FILE]+1 < MAX_NR_GENS.