Patch series "Add swappiness argument to memory.reclaim", v6.

This patch proposes augmenting the memory.reclaim interface with a
swappiness=<val> argument that overrides the swappiness value for that
instance of proactive reclaim.

Userspace proactive reclaimers use the memory.reclaim interface to trigger
reclaim.  The memory.reclaim interface does not allow for any way to
effect the balance of file vs anon during proactive reclaim.  The only
approach is to adjust the vm.swappiness setting.  However, there are a few
reasons we look to control the balance of file vs anon during proactive
reclaim, separately from reactive reclaim:

* Swapout should be limited to manage SSD write endurance.  In near-OOM
  situations we are fine with lots of swap-out to avoid OOMs.  As these
  are typically rare events, they have relatively little impact on write
  endurance.  However, proactive reclaim runs continuously and so its
  impact on SSD write endurance is more significant.  Therefore it is
  desireable to control swap-out for proactive reclaim separately from
  reactive reclaim

* Some userspace OOM killers like systemd-oomd[1] support OOM killing on
  swap exhaustion.  This makes sense if the swap exhaustion is triggered
  due to reactive reclaim but less so if it is triggered due to proactive
  reclaim (e.g.  one could see OOMs when free memory is ample but anon is
  just particularly cold).  Therefore, it's desireable to have proactive
  reclaim reduce or stop swap-out before the threshold at which OOM
  killing occurs.

In the case of Meta's Senpai proactive reclaimer, we adjust vm.swappiness
before writes to memory.reclaim[2].  This has been in production for
nearly two years and has addressed our needs to control proactive vs
reactive reclaim behavior but is still not ideal for a number of reasons:

* vm.swappiness is a global setting, adjusting it can race/interfere
  with other system administration that wishes to control vm.swappiness. 
  In our case, we need to disable Senpai before adjusting vm.swappiness.

* vm.swappiness is stateful - so a crash or restart of Senpai can leave
  a misconfigured setting.  This requires some additional management to
  record the "desired" setting and ensure Senpai always adjusts to it.

With this patch, we avoid these downsides of adjusting vm.swappiness
globally.

Previously, this exact interface addition was proposed by Yosry[3].  In
response, Roman proposed instead an interface to specify precise
file/anon/slab reclaim amounts[4].  More recently Huan also proposed this
as well[5] and others similarly questioned if this was the proper
interface.

Previous proposals sought to use this to allow proactive reclaimers to
effectively perform a custom reclaim algorithm by issuing proactive
reclaim with different settings to control file vs anon reclaim (e.g.  to
only reclaim anon from some applications).  Responses argued that
adjusting swappiness is a poor interface for custom reclaim.

In contrast, I argue in favor of a swappiness setting not as a way to
implement custom reclaim algorithms but rather to bias the balance of anon
vs file due to differences of proactive vs reactive reclaim.  In this
context, swappiness is the existing interface for controlling this balance
and this patch simply allows for it to be configured differently for
proactive vs reactive reclaim.

Specifying explicit amounts of anon vs file pages to reclaim feels
inappropriate for this prupose.  Proactive reclaimers are un-aware of the
relative age of file vs anon for a cgroup which makes it difficult to
manage proactive reclaim of different memory pools.  A proactive reclaimer
would need some amount of anon reclaim attempts separate from the amount
of file reclaim attempts which seems brittle given that it's difficult to
observe the impact.

[1]https://www.freedesktop.org/software/systemd/man/latest/systemd-oomd.service.html
[2]https://github.com/facebookincubator/oomd/blob/main/src/oomd/plugins/Senpai.cpp#L585-L598
[3]https://lore.kernel.org/linux-mm/CAJD7tkbDpyoODveCsnaqBBMZEkDvshXJmNdbk51yKSNgD7aGdg@mail.gmail.com/
[4]https://lore.kernel.org/linux-mm/YoPHtHXzpK51F%2F1Z@carbon/
[5]https://lore.kernel.org/lkml/20231108065818.19932-1-link@vivo.com/


This patch (of 2):

We use the constants 0 and 200 in a few places in the mm code when
referring to the min and max swappiness.  This patch adds MIN_SWAPPINESS
and MAX_SWAPPINESS #defines to improve clarity.  There are no functional
changes.

Link: https://lkml.kernel.org/r/20240103164841.2800183-1-schatzberg.dan@gmail.com
Link: https://lkml.kernel.org/r/20240103164841.2800183-2-schatzberg.dan@gmail.comSigned-off-by: Dan Schatzberg <schatzberg.dan@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Yue Zhao <findns94@gmail.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>