linux-kernel - Re: [PATCH] mempolicy: Clarify what RECLAIM

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Message-ID: <87ms8p2iop.fsf@DESKTOP-5N7EMDA>
Date: Mon, 28 Jul 2025 09:44:06 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
To: Joshua Hahn <joshua.hahnjy@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,  David Hildenbrand
 <david@...hat.com>,  Johannes Weiner <hannes@...xchg.org>,  Zi Yan
 <ziy@...dia.com>,  Matthew Brost <matthew.brost@...el.com>,  Rakie Kim
 <rakie.kim@...com>,  Byungchul Park <byungchul@...com>,  Gregory Price
 <gourry@...rry.net>,  Alistair Popple <apopple@...dia.com>,
  linux-kernel@...r.kernel.org,  linux-mm@...ck.org,  kernel-team@...a.com
Subject: Re: [PATCH] mempolicy: Clarify what RECLAIM_ZONE means

Hi, Joshua,

Joshua Hahn <joshua.hahnjy@...il.com> writes:

> The zone_reclaim_mode API controls reclaim behavior when a node runs out of
> memory. Contrary to its user-facing name, it is internally referred to as
> "node_reclaim_mode". This is slightly confusing but there is not much we can
> do given that it has already been exposed to userspace (since at least 2.6).
>
> However, what we can do is to make sure the internal description of what the
> bits inside zone_reclaim_mode aligns with what it does in practice.
> Setting RECLAIM_ZONE does indeed run shrink_inactive_list, but a more holistic
> description would be to explain that zone reclaim modulates whether page
> allocation (and khugepaged collapsing) prefers reclaiming & attempting to
> allocate locally or should fall back to the next node in the zonelist.
>
> Change the description to clarify what zone reclaim entails.
>
> Signed-off-by: Joshua Hahn <joshua.hahnjy@...il.com>
> ---
>  include/uapi/linux/mempolicy.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/mempolicy.h b/include/uapi/linux/mempolicy.h
> index 1f9bb10d1a47..24083809d920 100644
> --- a/include/uapi/linux/mempolicy.h
> +++ b/include/uapi/linux/mempolicy.h
> @@ -69,7 +69,7 @@ enum {
>   * These bit locations are exposed in the vm.zone_reclaim_mode sysctl
>   * ABI.  New bits are OK, but existing bits can never change.
>   */
> -#define RECLAIM_ZONE	(1<<0)	/* Run shrink_inactive_list on the zone */
> +#define RECLAIM_ZONE	(1<<0)	/* Prefer reclaiming & allocating locally */
>  #define RECLAIM_WRITE	(1<<1)	/* Writeout pages during reclaim */
>  #define RECLAIM_UNMAP	(1<<2)	/* Unmap pages during reclaim */
>  
>
> base-commit: 25fae0b93d1d7ddb25958bcb90c3c0e5e0e202bd

Please consider the document of zone_reclaim_mode in
Documentation/admin-guide/sysctl/vm.rst too.

And, IIUC, RECLAIM_ZONE doesn't mean "locally" exactly.  It's legal to
bind to some node other than "local node".

---
Best Regards,
Huang, Ying