linux-ext4 - Re: start removing writepage instances

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Date:   Fri, 2 Dec 2022 11:22:45 +0100
From:   Christoph Hellwig <hch@....de>
To:     "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
Cc:     Christoph Hellwig <hch@....de>,
        Namjae Jeon <linkinjeon@...nel.org>,
        Sungjong Seo <sj1557.seo@...sung.com>,
        Jan Kara <jack@...e.com>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>,
        Dave Kleikamp <shaggy@...nel.org>,
        Bob Copeland <me@...copeland.com>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        jfs-discussion@...ts.sourceforge.net,
        linux-karma-devel@...ts.sourceforge.net, linux-mm@...ck.org
Subject: Re: start removing writepage instances

On Thu, Nov 17, 2022 at 12:09:00AM +0530, Ritesh Harjani (IBM) wrote:
>    reclaim. Now IIUC from previous discussions [1][2][3], reclaims happens from
>    the tail end of the LRU list which could do an I/O of a single page while 
>    an ongoing writeback was in progress of multiple pages. This disrupts the I/O 
>    pattern to become more random in nature, compared to, if we would have let 
>    writeback/flusher do it's job of writing back dirty pages.

Yes.

>    Also many filesystems behave very differently within their ->writepage calls,
>    e.g. ext4 doesn't actually write in ->writepage for DELAYED blocks.

I don't think it's many file systems.  As far as I can tell only ext4
actually is significantly different.

> 2. Now the other place from where ->writepage can be called from is, writeout()
>    function, which is a fallback function for migration (fallback_migrate_folio()).
>    fallback_migrate_folio() is called from move_to_new_folio() if ->migrate_folio 
>    is not defined for the FS.

Also there is generic_writepages and folio_write_one/write_one_page.

> Is above a correct understanding?

Yes.