linux-ext4 - Re: [PATCH v3 11/12] ext4: Stop providing .writepage hook

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Date:   Mon, 5 Dec 2022 22:17:26 -0500
From:   "Theodore Ts'o" <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org
Subject: Re: [PATCH v3 11/12] ext4: Stop providing .writepage hook

On Mon, Dec 05, 2022 at 01:29:25PM +0100, Jan Kara wrote:
> Now we don't need .writepage hook for anything anymore. Reclaim is fine
> with relying on .writepages to clean pages and we often couldn't do much
> from the .writepage callback anyway. We only need to provide
> .migrate_folio callback for the ext4_journalled_aops - let's use
> buffer_migrate_page_norefs() there so that buffers cannot be modified
  ^^^^^^^^^^^^^^^^^^^^^^^^^^  this should be buffer_migrate_folio_norefs, no?
> under jdb2's hands.
> 
> Reviewed-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Jan Kara <jack@...e.cz>

Could you clarify in the commit how critical it is to use the
_norefs() variant?  It's not entirely clear what you mean by "let's
use...".  I think what is meant is that we need to use ..._noref() or
we can get in trouble if while the page update is getting committed,
there is an attempted to migrate the folio containing the page.

buffer_migrate_folio_norefs() is currently not exported (although
buffer_migrate_folio is).  So if we need it for ext4, we're going to
have to EXPORT_SYMBOL buffer_migrate_folio_norefs.

Any objections from the mm folks?

				- Ted