Re: [PATCH v3] btrfs: implement migratepage callback

[David Sterba]

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@xxxxxxxxx>
> 
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Reviewed-by: Chris Mason <clm@xxxxxx>

Added to devel queue, thanks.