Re: [RFC] some page can't be migrated

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Wednesday 23 January 2008 17:22, Shaohua Li wrote:
> Anonymous page might have fs-private metadata, the page is truncated. As
> the page hasn't mapping, page migration refuse to migrate the page. It
> appears the page is only freed in page reclaim and if zone watermark is
> low, the page is never freed, as a result migration always fail. I
> thought we could free the metadata so such page can be freed in
> migration and make migration more reliable?

Anonymous pages should not have fs-private metadata.

Orphaned pages I guess you mean? They should not be accessable via
the pagecache or the page tables, so how do they keep tangling up
migration? Where/how is migration finding these pages?!


>
> Thanks,
> Shaohua
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6a207e8..6bc38f7 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -633,6 +633,17 @@ static int unmap_and_move(new_page_t get_new_page,
> unsigned long private, goto unlock;
>  		wait_on_page_writeback(page);
>  	}
> +
> +	/*
> +	 * See truncate_complete_page(). Anonymous page might have
> +	 * fs-private metadata, the page is truncated. Such page can't be
> +	 * migrated. Try to free metadata, so the page can be freed.
> +	 */
> +	if (!page->mapping && !PageAnon(page) && PagePrivate(page)) {
> +		try_to_release_page(page, GFP_KERNEL);
> +		goto unlock;
> +	}
> +
>  	/*
>  	 * By try_to_unmap(), page->mapcount goes down to 0 here. In this case,
>  	 * we cannot notice that anon_vma is freed while we migrates a page.
>
>
> --