linux-kernel - Re: [PATCH v3] btrfs: implement migratepage callback

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Message-ID: <20200311004012.GC12659@twin.jikos.cz>
Date:   Wed, 11 Mar 2020 01:40:12 +0100
From:   David Sterba <dsterba@...e.cz>
To:     Roman Gushchin <guro@...com>
Cc:     Josef Bacik <josef@...icpanda.com>, Chris Mason <clm@...com>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v3] btrfs: implement migratepage callback

On Wed, Mar 04, 2020 at 04:57:35PM -0800, Roman Gushchin wrote:
> Currently btrfs doesn't provide a migratepage callback. It means that
> fallback_migrate_page()	is used to migrate btrfs pages.

The callback exists for the metadata pages (btree_migratepage), so I've
added 'for data pages' where appropriate.

> fallback_migrate_page() cannot move dirty pages, instead it tries to
> flush them (in sync mode) or just fails (in async mode).
> 
> In the sync mode pages which are scheduled to be processed by
> btrfs_writepage_fixup_worker() can't be effectively flushed by the
> migration code, because there is no established way to wait for the
> completion of the delayed work.
> 
> It all leads to page migration failures.
> 
> To fix it the patch implements a btrs-specific migratepage callback,
> which is similar to iomap_migrate_page() used by some other fs, except
> it does take care of the PagePrivate2 flag which is used for data
> ordering purposes.
> 
> v3: fixed the build issue once again
> v2: fixed the build issue found by the kbuild test robot <lkp@...el.com>
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> Reviewed-by: Chris Mason <clm@...com>

Added to devel queue, thanks.