linux-ext4 - generic_writepages & jbd2 and ext4

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Message-ID: <20221116135016.GA9713@lst.de>
Date:   Wed, 16 Nov 2022 14:50:16 +0100
From:   Christoph Hellwig <hch@....de>
To:     Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Mingming Cao <cmm@...ibm.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        ocfs2-devel@....oracle.com, linux-mm@...ck.org
Subject: generic_writepages & jbd2 and ext4

Hi all,

I've recently started looking into killing off the ->writepage method,
and as an initial subproject kill of external uses of generic_writepages.
One of the two remaining callers s in jbd2 and I'm a bit confused about
it.

jbd2_journal_submit_inode_data_buffers has two comments that explicitly
ask for ->writepages as that doesn't allocate data:

/*
 * write the filemap data using writepage() address_space_operations.
 * We don't do block allocation here even for delalloc. We don't
 * use writepages() because with delayed allocation we may be doing
 * block allocation in writepages().
 */

	/*
         * submit the inode data buffers. We use writepage
	 * instead of writepages. Because writepages can do
	 * block allocation with delalloc. We need to write
	 * only allocated blocks here.
	 */

and these look really stange to me.  ->writepage and ->writepages per
their document VM/VFS semantics don't different on what they allocate,
so this seems to reverse engineer ext4 internal behavior in some
way.  Either way looping over ->writepage just for that is rather
inefficient.  If jbd2 really wants a way to skip delalloc conversion
can we come up with a flag in struct writeback_control for that?

Is there anyone familiar enough with this code who would be willing
to give it a try?