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

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Date:   Mon, 13 May 2019 21:36:59 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
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, 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.

> than "it should be fine". This should have happened when THP swap out
> was implemented...
>
> --
> Michal Hocko
> SUSE Labs
>