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

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Message-ID: <20250728145109.1524733-1-joshua.hahnjy@gmail.com>
Date: Mon, 28 Jul 2025 07:51:07 -0700
From: Joshua Hahn <joshua.hahnjy@...il.com>
To: "Huang, Ying" <ying.huang@...ux.alibaba.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

On Mon, 28 Jul 2025 09:44:06 +0800 "Huang, Ying" <ying.huang@...ux.alibaba.com> wrote:

> 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

Hi Ying, thanks for your review, as always!

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

Yes, will do. Along with SJ's comment, I think that the information in the
admin-guide should be sufficient enough to explain what these bits do, so
I think my patch is not very necessary.

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

You are correct, it seems you can also reclaim on non-local nodes once you
go further down in the zonelist. I think my intent with the new comment was just
to indicate a preference to reclaim and allocate on the *current* node, as
opposed to falling back to the next node in the zonelist.

With that said, I think your comment along with SJ's feedback have gotten me
to understand that we proably don't need this change : -) 

Thank you, and have a great day!
Joshua

> ---
> Best Regards,
> Huang, Ying

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