linux-kernel - Re: [PATCH] mm: vmscan: do not pass reclaimed slab to vmpressure

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Date:   Wed, 7 Jun 2017 11:53:24 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     zhong jiang <zhongjiang@...wei.com>
CC:     vinayak menon <vinayakm.list@...il.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        <mgorman@...hsingularity.net>, <vbabka@...e.cz>, <mhocko@...e.com>,
        Rik van Riel <riel@...hat.com>, <vdavydov.dev@...il.com>,
        <anton.vorontsov@...aro.org>,
        Shiraz Hashim <shiraz.hashim@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: vmscan: do not pass reclaimed slab to vmpressure

Hi,

On Tue, Jun 06, 2017 at 09:00:55PM +0800, zhong jiang wrote:
> On 2017/1/31 7:40, Minchan Kim wrote:
> > Hi Vinayak,
> > Sorry for late response. It was Lunar New Year holidays.
> >
> > On Fri, Jan 27, 2017 at 01:43:23PM +0530, vinayak menon wrote:
> >>> Thanks for the explain. However, such case can happen with THP page
> >>> as well as slab. In case of THP page, nr_scanned is 1 but nr_reclaimed
> >>> could be 512 so I think vmpressure should have a logic to prevent undeflow
> >>> regardless of slab shrinking.
> >>>
> >> I see. Going to send a vmpressure fix. But, wouldn't the THP case
> >> result in incorrect
> >> vmpressure reporting even if we fix the vmpressure underflow problem ?
> > If a THP page is reclaimed, it reports lower pressure due to bigger
> > reclaim ratio(ie, reclaimed/scanned) compared to normal pages but
> > it's not a problem, is it? Because VM reclaimed more memory than
> > expected so memory pressure isn't severe now.
>   Hi, Minchan
> 
>   THP lru page is reclaimed, reclaim ratio bigger make sense. but I read the code, I found
>   THP is split to normal pages and loop again.  reclaimed pages should not be bigger
>    than nr_scan.  because of each loop will increase nr_scan counter.
>  
>    It is likely  I miss something.  you can point out the point please.

You are absolutely right.

I got confused by nr_scanned from isolate_lru_pages and sc->nr_scanned
from shrink_page_list.

Thanks.