Re: [v2 PATCH] mm: vmscan: correct nr_reclaimed for THP

[Yang Shi]

On 5/13/19 11:20 PM, Michal Hocko wrote:
> On Mon 13-05-19 21:36:59, Yang Shi wrote:
> > On Mon, May 13, 2019 at 2:45 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > > On Mon 13-05-19 14:09:59, Yang Shi wrote:
> > > [...]
> > > > I think we can just account 512 base pages for nr_scanned for
> > > > isolate_lru_pages() to make the counters sane since PGSCAN_KSWAPD/DIRECT
> > > > just use it.
> > > >
> > > > And, sc->nr_scanned should be accounted as 512 base pages too otherwise we
> > > > may have nr_scanned < nr_to_reclaim all the time to result in false-negative
> > > > for priority raise and something else wrong (e.g. wrong vmpressure).
> > > Be careful. nr_scanned is used as a pressure indicator to slab shrinking
> > > AFAIR. Maybe this is ok but it really begs for much more explaining
> > I don't know why my company mailbox didn't receive this email, so I
> > replied with my personal email.
> >
> > It is not used to double slab pressure any more since commit
> > 9092c71bb724 ("mm: use sc->priority for slab shrink targets"). It uses
> > sc->priority to determine the pressure for slab shrinking now.
> >
> > So, I think we can just remove that "double slab pressure" code. It is
> > not used actually and looks confusing now. Actually, the "double slab
> > pressure" does something opposite. The extra inc to sc->nr_scanned
> > just prevents from raising sc->priority.
> I have to get in sync with the recent changes. I am aware there were
> some patches floating around but I didn't get to review them. I was
> trying to point out that nr_scanned used to have a side effect to be
> careful about. If it doesn't have anymore then this is getting much more
> easier of course. Please document everything in the changelog.

Thanks for reminding. Yes, I remembered nr_scanned would double slab 
pressure. But, when I inspected into the code yesterday, it turns out it 
is not true anymore. I will run some test to make sure it doesn't 
introduce regression.

BTW, I noticed the counter of memory reclaim is not correct with THP 
swap on vanilla kernel, please see the below:

pgsteal_kswapd 21435
pgsteal_direct 26573329
pgscan_kswapd 3514
pgscan_direct 14417775

pgsteal is always greater than pgscan, my patch could fix the problem.

Anyway, I will elaborate these in the commit log.