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Date: Tue, 4 Apr 2023 14:38:24 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
"Darrick J. Wong" <djwong@...nel.org>,
Christoph Lameter <cl@...ux.com>,
David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Vlastimil Babka <vbabka@...e.cz>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Hyeonggon Yoo <42.hyeyoo@...il.com>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>,
Miaohe Lin <linmiaohe@...wei.com>,
David Hildenbrand <david@...hat.com>,
Johannes Weiner <hannes@...xchg.org>,
Peter Xu <peterx@...hat.com>, NeilBrown <neilb@...e.de>,
Shakeel Butt <shakeelb@...gle.com>,
Michal Hocko <mhocko@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
Dave Chinner <david@...morbit.com>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-xfs@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 0/3] Ignore non-LRU-based reclaim in memcg reclaim
On Tue, 4 Apr 2023 00:13:50 +0000 Yosry Ahmed <yosryahmed@...gle.com> wrote:
> Upon running some proactive reclaim tests using memory.reclaim, we
> noticed some tests flaking where writing to memory.reclaim would be
> successful even though we did not reclaim the requested amount fully.
> Looking further into it, I discovered that *sometimes* we over-report
> the number of reclaimed pages in memcg reclaim.
>
> Reclaimed pages through other means than LRU-based reclaim are tracked
> through reclaim_state in struct scan_control, which is stashed in
> current task_struct. These pages are added to the number of reclaimed
> pages through LRUs. For memcg reclaim, these pages generally cannot be
> linked to the memcg under reclaim and can cause an overestimated count
> of reclaimed pages. This short series tries to address that.
>
> Patches 1-2 are just refactoring, they add helpers that wrap some
> operations on current->reclaim_state, and rename
> reclaim_state->reclaimed_slab to reclaim_state->reclaimed.
>
> Patch 3 ignores pages reclaimed outside of LRU reclaim in memcg reclaim.
> The pages are uncharged anyway, so even if we end up under-reporting
> reclaimed pages we will still succeed in making progress during
> charging.
>
> Do not let the diff stat deceive you, the core of this series is patch 3,
> which has one line of code change. All the rest is refactoring and one
> huge comment.
>
Wouldn't it be better to do this as a single one-line patch for
backportability? Then all the refactoring etcetera can be added on
later.
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