linux-kernel - Re: [RESEND PATCH v7 6/8] mm: Add APIs to free a folio directly to the buddy bypassing pcp

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Message-ID: <20240213020307.GC4147@system.software.com>
Date: Tue, 13 Feb 2024 11:03:07 +0900
From: Byungchul Park <byungchul@...com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kernel_team@...ynix.com, ying.huang@...el.com, namit@...are.com,
	vernhao@...cent.com, mgorman@...hsingularity.net, hughd@...gle.com,
	willy@...radead.org, david@...hat.com, peterz@...radead.org,
	luto@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, rjgolo@...il.com
Subject: Re: [RESEND PATCH v7 6/8] mm: Add APIs to free a folio directly to
 the buddy bypassing pcp

On Thu, Feb 08, 2024 at 12:49:19PM -0800, Andrew Morton wrote:
> On Thu,  8 Feb 2024 15:26:06 +0900 Byungchul Park <byungchul@...com> wrote:
> 
> > This is a preparation for migrc mechanism that frees folios at a better
> 
> The term "migrc" appears in various places but I don't think we're told
> what is actually means?
> 
> > time later, rather than the moment migrating folios. The folios freed by
> > migrc are too old to keep in pcp.
> 
> How do we define "too old" and what causes you to believe this is the case?

Migrc defers folio_put() for source folios of migration that would be
unlikely used and frees a bunch of folios at once later. However, it
pollutes pcp, which means fresher folios might get free_pcppages_bulk()ed
and makes the effort to keep the best amount of pcp get unstable. So I
didn't want to make this situation happen.

	Byungchul