linux-kernel - Re: [v2 PATCH] mm: vmscan: correct nr

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Date:   Tue, 14 May 2019 08:20:39 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <shy828301@...il.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>,
        Huang Ying <ying.huang@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        kirill.shutemov@...ux.intel.com, Hugh Dickins <hughd@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        william.kucharski@...cle.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v2 PATCH] mm: vmscan: correct nr_reclaimed for THP

On Mon 13-05-19 21:36:59, Yang Shi wrote:
> On Mon, May 13, 2019 at 2:45 PM Michal Hocko <mhocko@...nel.org> 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.
-- 
Michal Hocko
SUSE Labs