Message-ID: <6aba66e6-0bc5-4afb-a42c-a85756716e1c@lucifer.local>
Date: Mon, 30 Jun 2025 18:05:40 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [PATCH v1 09/29] mm/migrate: factor out movable_ops page
 handling into migrate_movable_ops_page()

On Mon, Jun 30, 2025 at 02:59:50PM +0200, David Hildenbrand wrote:
> Let's factor it out, simplifying the calling code.
>
> The assumption is that flush_dcache_page() is not required for
> movable_ops pages: as documented for flush_dcache_folio(), it really
> only applies when the kernel wrote to pagecache pages / pages in
> highmem. movable_ops callbacks should be handling flushing
> caches if ever required.

But we've enot changed this have we? The flush_dcache_folio() invocation seems
to happen the same way now as before? Did I miss something?

>
> Note that we can now change folio_mapping_flags() to folio_test_anon()
> to make it clearer, because movable_ops pages will never take that path.
>
> Reviewed-by: Zi Yan <ziy@...dia.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Have scrutinised this a lot and it seems correct to me, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  mm/migrate.c | 82 ++++++++++++++++++++++++++++------------------------
>  1 file changed, 45 insertions(+), 37 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index d97f7cd137e63..0898ddd2f661f 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -159,6 +159,45 @@ static void putback_movable_ops_page(struct page *page)
>  	folio_put(folio);
>  }
>
> +/**
> + * migrate_movable_ops_page - migrate an isolated movable_ops page
> + * @page: The isolated page.
> + *
> + * Migrate an isolated movable_ops page.
> + *
> + * If the src page was already released by its owner, the src page is
> + * un-isolated (putback) and migration succeeds; the migration core will be the
> + * owner of both pages.
> + *
> + * If the src page was not released by its owner and the migration was
> + * successful, the owner of the src page and the dst page are swapped and
> + * the src page is un-isolated.
> + *
> + * If migration fails, the ownership stays unmodified and the src page
> + * remains isolated: migration may be retried later or the page can be putback.
> + *
> + * TODO: migration core will treat both pages as folios and lock them before
> + * this call to unlock them after this call. Further, the folio refcounts on
> + * src and dst are also released by migration core. These pages will not be
> + * folios in the future, so that must be reworked.
> + *
> + * Returns MIGRATEPAGE_SUCCESS on success, otherwise a negative error
> + * code.
> + */

Love these comments you're adding!!

> +static int migrate_movable_ops_page(struct page *dst, struct page *src,
> +		enum migrate_mode mode)
> +{
> +	int rc = MIGRATEPAGE_SUCCESS;

Maybe worth asserting src, dst locking?

> +
> +	VM_WARN_ON_ONCE_PAGE(!PageIsolated(src), src);
> +	/* If the page was released by it's owner, there is nothing to do. */
> +	if (PageMovable(src))
> +		rc = page_movable_ops(src)->migrate_page(dst, src, mode);
> +	if (rc == MIGRATEPAGE_SUCCESS)
> +		ClearPageIsolated(src);
> +	return rc;
> +}
> +
>  /*
>   * Put previously isolated pages back onto the appropriate lists
>   * from where they were once taken off for compaction/migration.
> @@ -1023,51 +1062,20 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>  								mode);
>  		else
>  			rc = fallback_migrate_folio(mapping, dst, src, mode);
> -	} else {
> -		const struct movable_operations *mops;
>
> -		/*
> -		 * In case of non-lru page, it could be released after
> -		 * isolation step. In that case, we shouldn't try migration.
> -		 */
> -		VM_BUG_ON_FOLIO(!folio_test_isolated(src), src);
> -		if (!folio_test_movable(src)) {
> -			rc = MIGRATEPAGE_SUCCESS;
> -			folio_clear_isolated(src);
> +		if (rc != MIGRATEPAGE_SUCCESS)
>  			goto out;
> -		}
> -
> -		mops = folio_movable_ops(src);
> -		rc = mops->migrate_page(&dst->page, &src->page, mode);
> -		WARN_ON_ONCE(rc == MIGRATEPAGE_SUCCESS &&
> -				!folio_test_isolated(src));
> -	}
> -
> -	/*
> -	 * When successful, old pagecache src->mapping must be cleared before
> -	 * src is freed; but stats require that PageAnon be left as PageAnon.
> -	 */
> -	if (rc == MIGRATEPAGE_SUCCESS) {
> -		if (__folio_test_movable(src)) {
> -			VM_BUG_ON_FOLIO(!folio_test_isolated(src), src);
> -
> -			/*
> -			 * We clear PG_movable under page_lock so any compactor
> -			 * cannot try to migrate this page.
> -			 */
> -			folio_clear_isolated(src);
> -		}
> -
>  		/*
> -		 * Anonymous and movable src->mapping will be cleared by
> -		 * free_pages_prepare so don't reset it here for keeping
> -		 * the type to work PageAnon, for example.
> +		 * For pagecache folios, src->mapping must be cleared before src
> +		 * is freed. Anonymous folios must stay anonymous until freed.
>  		 */
> -		if (!folio_mapping_flags(src))
> +		if (!folio_test_anon(src))
>  			src->mapping = NULL;
>
>  		if (likely(!folio_is_zone_device(dst)))
>  			flush_dcache_folio(dst);
> +	} else {
> +		rc = migrate_movable_ops_page(&dst->page, &src->page, mode);
>  	}
>  out:
>  	return rc;
> --
> 2.49.0
>

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