linux-kernel - Re: [PATCH 06/10] mm: vmscan: demote anon DRAM pages to PMEM node

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Date:   Wed, 27 Mar 2019 07:08:22 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     "mhocko@...e.com" <mhocko@...e.com>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "riel@...riel.com" <riel@...riel.com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Busch, Keith" <keith.busch@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Wu, Fengguang" <fengguang.wu@...el.com>,
        "Du, Fan" <fan.du@...el.com>, "Huang, Ying" <ying.huang@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/10] mm: vmscan: demote anon DRAM pages to PMEM node

On Tue, Mar 26, 2019 at 08:41:15PM -0700, Yang Shi wrote:
> On 3/26/19 5:35 PM, Keith Busch wrote:
> > migration nodes have higher free capacity than source nodes. And since
> > your attempting THP's without ever splitting them, that also requires
> > lower fragmentation for a successful migration.
> 
> Yes, it is possible. However, migrate_pages() already has logic to 
> handle such case. If the target node has not enough space for migrating 
> THP in a whole, it would split THP then retry with base pages.

Oh, you're right, my mistake on splitting. So you have a good best effort
migrate, but I still think it can fail for legitimate reasons that should
have a swap fallback.