linux-ext4 - Re: [PATCH 15/20] balloon: Convert to migrate

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Date:   Wed, 8 Jun 2022 09:06:17 -0400
From:   Rafael Aquini <aquini@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-aio@...ck.org,
        linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        linux-mm@...ck.org, linux-xfs@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-ntfs-dev@...ts.sourceforge.net,
        ocfs2-devel@....oracle.com, linux-mtd@...ts.infradead.org,
        virtualization@...ts.linux-foundation.org,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH 15/20] balloon: Convert to migrate_folio

On Wed, Jun 08, 2022 at 11:59:31AM +0200, David Hildenbrand wrote:
> On 07.06.22 21:21, Matthew Wilcox wrote:
> > On Tue, Jun 07, 2022 at 03:24:15PM +0100, Matthew Wilcox wrote:
> >> On Tue, Jun 07, 2022 at 09:36:21AM +0200, David Hildenbrand wrote:
> >>> On 06.06.22 22:40, Matthew Wilcox (Oracle) wrote:
> >>>>  const struct address_space_operations balloon_aops = {
> >>>> -	.migratepage = balloon_page_migrate,
> >>>> +	.migrate_folio = balloon_migrate_folio,
> >>>>  	.isolate_page = balloon_page_isolate,
> >>>>  	.putback_page = balloon_page_putback,
> >>>>  };
> >>>
> >>> I assume you're working on conversion of the other callbacks as well,
> >>> because otherwise, this ends up looking a bit inconsistent and confusing :)
> >>
> >> My intention was to finish converting aops for the next merge window.
> >>
> >> However, it seems to me that we goofed back in 2016 by merging
> >> commit bda807d44454.  isolate_page() and putback_page() should
> >> never have been part of address_space_operations.
> >>
> >> I'm about to embark on creating a new migrate_operations struct
> >> for drivers to use that contains only isolate/putback/migrate.
> >> No filesystem uses isolate/putback, so those can just be deleted.
> >> Both migrate_operations & address_space_operations will contain a
> >> migrate callback.
> 
> That makes sense to me. I wonder if there was a design
> decision/discussion behind that. CCing Rafael.
>

None that I recollect. If memory still serves me, I think the idea behind
bda807d44454 and friends was to provide a generic way to allow page
mobility for drivers without adding complexity to the page isolation / putback
paths, and since the migration callback was already part of the aops struct
those new callbacks just followed suit.


-- Rafael