linux-kernel - Re: [PATCH 1/4] migrate: Allow migrate_misplaced

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Message-ID: <Z03WrdqWV1NBf1NA@PC2K9PVX.TheFacebook.com>
Date: Mon, 2 Dec 2024 10:47:57 -0500
From: Gregory Price <gourry@...rry.net>
To: "Huang, Ying" <ying.huang@...ux.alibaba.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, nehagholkar@...a.com,
	abhishekd@...a.com, kernel-team@...a.com, david@...hat.com,
	ying.huang@...el.com, nphamcs@...il.com, akpm@...ux-foundation.org,
	hannes@...xchg.org, feng.tang@...el.com, kbusch@...a.com
Subject: Re: [PATCH 1/4] migrate: Allow migrate_misplaced_folio APIs without
 a VMA

On Thu, Nov 28, 2024 at 07:12:11PM +0800, Huang, Ying wrote:
> Hi, Gregory,
> 
> Gregory Price <gourry@...rry.net> writes:
> 
> > To migrate unmapped pagecache folios, migrate_misplaced_folio and
> > migrate_misplaced_folio_prepare must handle folios without VMAs.
> 
> IMHO, it's better to use migrate_misplaced_folio() instead of
> migrate_misplaced_folio for readability in patch title and description.
> 
> > migrate_misplaced_folio_prepare checks VMA for exec bits, so allow
> > a NULL VMA when it does not have a mapping.
> >
> > migrate_misplaced_folio must call migrate_pages with MIGRATE_SYNC
> > when in the pagecache path because it is a synchronous context.
> 
> I don't find the corresponding implementation for this.  And, I don't
> think it's a good idea to change from MIGRATE_ASYNC to MIGRATE_SYNC.
> This may cause too long page access latency for page placement
> optimization.  The downside may offset the benefit.
> 
> And, it appears that we can delete the "vma" parameter of
> migrate_misplaced_folio() because it's not used now.  This is a trivial
> code cleanup.
>

This patch apparently got a bit away from me and was heavily reduced
from its initial form.  This commit message is just wrong now.  I will
update this and the 2nd commit and probably submit them separately.

~Gregory