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Subject: Re: [v2 PATCH] mm: vmscan: correct nr_reclaimed for THP
To:     Michal Hocko <mhocko@kernel.org>, Yang Shi <shy828301@gmail.com>
Cc:     Huang Ying <ying.huang@intel.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        kirill.shutemov@linux.intel.com, Hugh Dickins <hughd@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        william.kucharski@oracle.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <509de066-17bb-e3cf-d492-1daf1cb11494@linux.alibaba.com>
Date:   Tue, 14 May 2019 13:44:35 -0700
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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@kernel.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.

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.