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Subject: Re: [PATCH v3] vm_swappiness=0 should still try to avoid swapping
 anon memory
To:     Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>
Cc:     Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Rafael Aquini <aquini@redhat.com>,
        Waiman Long <llong@redhat.com>, Michal Hocko <mhocko@suse.com>,
        hakavlad@inbox.lv
References: <20210809223740.59009-1-npache@redhat.com>
 <YRKa0yzEDALtvSZO@cmpxchg.org>
 <a7afc485-9036-8f09-e582-f2eb45620670@redhat.com>
 <CALvZod5L+cyON5qRX3eSdHbo+-H7VDQeFuYYNqpK5diCcv9OMw@mail.gmail.com>
From:   Nico Pache <npache@redhat.com>
Message-ID: <cbdbeae9-010e-ca11-eeab-44fa7d78c83a@redhat.com>
Date:   Tue, 10 Aug 2021 18:16:33 -0400
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On 8/10/21 5:17 PM, Shakeel Butt wrote:
> On Tue, Aug 10, 2021 at 12:24 PM Nico Pache <npache@redhat.com> wrote:
>>
>>
> [...]
>>>
>>> I'm wondering how you're getting anon scans with swappiness=0. If you
>>> look at get_scan_count(), SCAN_FRACT with swappines=0 should always
>>> result in ap = fraction[0] = 0, which never yields any anon scan
>>> targets. So I'm thinking you're running into sc->file_is_tiny
>>> situations, meaning remaining file pages alone are not enough to
>>> restore watermarks anymore. Is that possible?
>>
>> Yes DEACTIVATE_ANON is enabling the file_is_tiny case in shrink_node(). That is what im trying to prevent in the swappiness=0 case.
>>
> 
> Can you please explain how DEACTIVATE_ANON is enabling the file_is_tiny case?


You're right. Just did a second pass... I misinterpreted the assignment to
file_is_tiny. This is not the case that is causing the issue. So back to the
SCAN_FRACT case. From my testing the refaulting still seems to be causing the
issue; however, to your point in earlier discussions, if swappiness=0 then the
get_scan_count *should* be 0.
So my patch does solve the issue by preventing the shrink_list from deactivating
the anon, but it may be hiding some other issue that is the ultimate cause.

Thanks for pointing that out!
-- Nico