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

[Minchan Kim]

On Tue, Apr 05, 2016 at 02:03:05PM +0200, Vlastimil Babka wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxxxxxx
> >Cc: Rafael Aquini <aquini@xxxxxxxxxx>
> >Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
> >Signed-off-by: Gioh Kim <gurugio@xxxxxxxxxxx>
> >Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> 
> 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?

Thanks for pointing out.
I will not use page->_mapcount in next version so it should be okay.

> 
> 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().

Yeb.

> 
> >@@ -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()?

Fixed.

> 
> >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.

I will change it next version so it shouldn't be a problem.
Thanks for the review!