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Date:   Thu, 30 Nov 2023 16:46:30 -0500
From:   Dan Schatzberg <schatzberg.dan@gmail.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Yosry Ahmed <yosryahmed@google.com>, Huan Yang <link@vivo.com>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org, Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Huang Ying <ying.huang@intel.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Peter Xu <peterx@redhat.com>,
        "Vishal Moola (Oracle)" <vishal.moola@gmail.com>,
        Yue Zhao <findns94@gmail.com>, Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH 1/1] mm: add swapiness= arg to memory.reclaim
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On Thu, Nov 30, 2023 at 01:33:40PM -0800, Andrew Morton wrote:
> On Thu, 30 Nov 2023 07:36:54 -0800 Dan Schatzberg <schatzberg.dan@gmail.com> wrote:
> 
> > Allow proactive reclaimers to submit an additional swappiness=<val>
> > argument to memory.reclaim. This overrides the global or per-memcg
> > swappiness setting for that reclaim attempt.
> > 
> > For example:
> > 
> > echo "2M swappiness=0" > /sys/fs/cgroup/memory.reclaim
> > 
> > will perform reclaim on the rootcg with a swappiness setting of 0 (no
> > swap) regardless of the vm.swappiness sysctl setting.
> > 
> > Signed-off-by: Dan Schatzberg <schatzberg.dan@gmail.com>
> > ---
> >  include/linux/swap.h |  3 ++-
> >  mm/memcontrol.c      | 55 +++++++++++++++++++++++++++++++++++---------
> >  mm/vmscan.c          | 13 +++++++++--
> 
> Documentation/admin-guide/cgroup-v2.rst is feeling unloved!

Oops - total oversight on my part. I'll add docs in a V2 if we can
come to consensus on this interface change in general.

> 
> Please check whether this interface change can lead to
> non-backward-compatible userspace.  If someone's script does the above
> echo command, will it break on older kernels?  If so, does it matter?

Older kernels will return -EINVAL with such a command - that seems
appropriate, indicating that the argument is not supported.