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

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Message-ID: <EE31DE08-36D3-4A5F-910D-9264FF973C30@fb.com>
Date:   Wed, 04 Mar 2020 14:53:21 -0500
From:   "Chris Mason" <clm@...com>
To:     Roman Gushchin <guro@...com>
CC:     Josef Bacik <josef@...icpanda.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] btrfs: implement migratepage callback



On 4 Mar 2020, at 14:50, Roman Gushchin wrote:

> Currently btrfs doesn't provide a migratepage callback. It means that
> fallback_migrate_page()	is used to migrate btrfs pages.
>
> 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.

Since the default migratepage didn’t copy PagePrivate2, didn’t you 
find it was also causing pages to get funneled into the fixup worker 
flow?

-chris