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From:   "Huang, Ying" <ying.huang@intel.com>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, yang.shi@linux.alibaba.com,
        rientjes@google.com, dan.j.williams@intel.com, david@redhat.com,
        osalvador@suse.de, weixugc@google.com
Subject: Re: [PATCH 00/10] [v7][RESEND] Migrate Pages in lieu of discard
References: <20210401183216.443C4443@viggo.jf.intel.com>
        <YHmEm/yHpaqO6khp@dhcp22.suse.cz>
        <9cd0dcde-f257-1b94-17d0-f2e24a3ce979@intel.com>
        <YHmm11fxEnxXAxas@dhcp22.suse.cz>
Date:   Wed, 21 Apr 2021 10:39:09 +0800
In-Reply-To: <YHmm11fxEnxXAxas@dhcp22.suse.cz> (Michal Hocko's message of
        "Fri, 16 Apr 2021 17:02:23 +0200")
Message-ID: <87czuo1hv6.fsf@yhuang6-desk1.ccr.corp.intel.com>
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Michal Hocko <mhocko@suse.com> writes:

> On Fri 16-04-21 07:26:43, Dave Hansen wrote:
>> On 4/16/21 5:35 AM, Michal Hocko wrote:
>> > - Anonymous pages are bit tricky because they can be demoted even when
>> >   they cannot be reclaimed due to no (or no available) swap storage.
>> >   Unless I have missed something the second round will try to reclaim
>> >   them even the later is true and I am not sure this is completely OK.
>> 
>> What we want is something like this:
>> 
>> Swap Space / Demotion OK  -> Can Reclaim
>> Swap Space / Demotion Off -> Can Reclaim
>> Swap Full  / Demotion OK  -> Can Reclaim
>> Swap Full  / Demotion Off -> No Reclaim
>> 
>> I *think* that's what can_reclaim_anon_pages() ends up doing.  Maybe I'm
>> misunderstanding what you are referring to, though.  By "second round"
>> did you mean when we do reclaim on a node which is a terminal node?
>
> No, I mean the migration failure case where you splice back to the page
> list to reclaim. In that round you do not demote and want to reclaim.
> But a lack of swap space will make that page unreclaimable. I suspect
> this would just work out fine but I am not sure from the top of my head.

I have tested this via injecting some migration errors (returning 0 from
demote_page_list() before migration) on a system without swap.  The
system can still work properly.  In ftrace, I can find add_to_swap() are
called much more times, and it can deal with the situation where the
swap space isn't available.

Best Regards,
Huang, Ying