Re: [PATCH v3 04/16] mm/balloon: use general movable page feature into balloon

Vlastimil Babka
2016-04-05T12:03:14Z

On 03/30/2016 09:12 AM, Minchan Kim wrote:
> Now, VM has a feature to migrate non-lru movable pages so
> balloon doesn't need custom migration hooks in migrate.c
> and compact.c. Instead, this patch implements page->mapping
> ->{isolate|migrate|putback} functions.
>
> With that, we could remove hooks for ballooning in general
> migration functions and make balloon compaction simple.
>
> Cc: virtualization@lists.linux-foundation.org
> Cc: Rafael Aquini <aquini@redhat.com>
> Cc: Konstantin Khlebnikov <koct9i@gmail.com>
> Signed-off-by: Gioh Kim <gurugio@hanmail.net>
> Signed-off-by: Minchan Kim <minchan@kernel.org>

I'm not familiar with the inode and pseudofs stuff, so just some things
I noticed:

> -#define PAGE_MOVABLE_MAPCOUNT_VALUE (-255)
> +#define PAGE_MOVABLE_MAPCOUNT_VALUE (-256)
> +#define PAGE_BALLOON_MAPCOUNT_VALUE PAGE_MOVABLE_MAPCOUNT_VALUE
>
> static inline int PageMovable(struct page *page)
> {
> - return ((test_bit(PG_movable, &(page)->flags) &&
> - atomic_read(&page->_mapcount) == PAGE_MOVABLE_MAPCOUNT_VALUE)
> - || PageBalloon(page));
> + return (test_bit(PG_movable, &(page)->flags) &&
> + atomic_read(&page->_mapcount) == PAGE_MOVABLE_MAPCOUNT_VALUE);
> }
>
> /* Caller should hold a PG_lock */
> @@ -645,6 +626,35 @@ static inline void __ClearPageMovable(struct page *page)
>
> PAGEFLAG(Isolated, isolated, PF_ANY);
>
> +static inline int PageBalloon(struct page *page)
> +{
> + return atomic_read(&page->_mapcount) == PAGE_BALLOON_MAPCOUNT_VALUE
> + && PagePrivate2(page);
> +}

Hmm so you are now using PG_private_2 flag here, but it's not
documented. Also the only caller of PageBalloon() seems to be
stable_page_flags(). Which will now report all movable pages with
PG_private_2 as KPF_BALOON. Seems like an overkill and also not
reliable. Could it test e.g. page->mapping instead?

Or maybe if we manage to get rid of PAGE_MOVABLE_MAPCOUNT_VALUE, we can
keep PAGE_BALLOON_MAPCOUNT_VALUE to simply distinguish balloon pages for
stable_page_flags().

> @@ -1033,7 +1019,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> out:
> /* If migration is successful, move newpage to right list */
> if (rc == MIGRATEPAGE_SUCCESS) {
> - if (unlikely(__is_movable_balloon_page(newpage)))
> + if (unlikely(PageMovable(newpage)))
> put_page(newpage);
> else
> putback_lru_page(newpage);

Hmm shouldn't the condition have been changed to

if (unlikely(__is_movable_balloon_page(newpage)) || PageMovable(newpage)

by patch 02/16? And this patch should be just removing the
balloon-specific check? Otherwise it seems like between patches 02 and
04, other kinds of PageMovable pages were unnecessarily/wrongly routed
through putback_lru_page()?

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d82196244340..c7696a2e11c7 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1254,7 +1254,7 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
>
> list_for_each_entry_safe(page, next, page_list, lru) {
> if (page_is_file_cache(page) && !PageDirty(page) &&
> - !isolated_balloon_page(page)) {
> + !PageIsolated(page)) {
> ClearPageActive(page);
> list_move(&page->lru, &clean_pages);
> }

This looks like the same comment as above at first glance. But looking
closer, it's even weirder. isolated_balloon_page() was simply
PageBalloon() after d6d86c0a7f8dd... weird already. You replace it with
check for !PageIsolated() which looks like a more correct check, so ok.
Except the potential false positive with PG_owner_priv_1.

Thanks.