linux-kernel - Re: [PATCH v6 5/9] mm: multigenerational lru: mm

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Message-ID: <YdgChw4vNb43XsU/@dhcp22.suse.cz>
Date:   Fri, 7 Jan 2022 10:06:15 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        page-reclaim@...gle.com, x86@...nel.org,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>
Subject: Re: [PATCH v6 5/9] mm: multigenerational lru: mm_struct list

On Tue 04-01-22 13:22:24, Yu Zhao wrote:
> To exploit spatial locality, the aging prefers to walk page tables to
> search for young PTEs. And this patch paves the way for that.
> 
> An mm_struct list is maintained for each memcg, and an mm_struct
> follows its owner task to the new memcg when this task is migrated.

How does this work actually for the memcg reclaim? I can see you
lru_gen_migrate_mm on the task migration. My concern is, though, that
such a task leaves all the memory behind in the previous memcg (in
cgroup v2, in v1 you can opt in for charge migration). If you move the
mm to a new memcg then you age it somewhere where the memory is not
really consumed.
-- 
Michal Hocko
SUSE Labs