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

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Date:	Tue, 5 Apr 2016 14:03:05 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jlayton@...chiereds.net, bfields@...ldses.org,
	Joonsoo Kim <iamjoonsoo.kim@....com>, koct9i@...il.com,
	aquini@...hat.com, virtualization@...ts.linux-foundation.org,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Rik van Riel <riel@...hat.com>, rknize@...orola.com,
	Gioh Kim <gi-oh.kim@...fitbricks.com>,
	Sangseok Lee <sangseok.lee@....com>,
	Chan Gyun Jeong <chan.jeong@....com>,
	Al Viro <viro@...IV.linux.org.uk>,
	YiPing Xu <xuyiping@...ilicon.com>,
	Gioh Kim <gurugio@...mail.net>
Subject: Re: [PATCH v3 04/16] mm/balloon: use general movable page feature
 into balloon

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@...ts.linux-foundation.org
> Cc: Rafael Aquini <aquini@...hat.com>
> Cc: Konstantin Khlebnikov <koct9i@...il.com>
> Signed-off-by: Gioh Kim <gurugio@...mail.net>
> Signed-off-by: Minchan Kim <minchan@...nel.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.