Re: [PATCH] mm: compaction: Abort compaction if too many pages areisolated and caller is asynchronous

[Minchan Kim]

On Thu, Jun 02, 2011 at 08:23:02PM +0200, Andrea Arcangeli wrote:
> On Tue, May 31, 2011 at 11:38:30PM +0900, Minchan Kim wrote:
> > > Yes. You find a new BUG.
> > > It seems to be related to this problem but it should be solved although
> > 
> >  typo : It doesn't seem to be.
> 
> This should fix it, but I doubt it matters for this problem.
> 
> ===
> Subject: mm: no page_count without a page pin
> 
> From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> 
> It's unsafe to run page_count during the physical pfn scan.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index faa0a08..e41e78a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1124,8 +1124,18 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  					nr_lumpy_dirty++;
>  				scan++;
>  			} else {
> -				/* the page is freed already. */
> -				if (!page_count(cursor_page))
> +				/*
> +				 * We can't use page_count() as that
> +				 * requires compound_head and we don't
> +				 * have a pin on the page here. If a
> +				 * page is tail, we may or may not
> +				 * have isolated the head, so assume
> +				 * it's not free, it'd be tricky to

Isn't it rather aggressive?
I think cursor page is likely to be PageTail rather than PageHead.
Could we handle it simply with below code?

get_page(cursor_page)
/* The page is freed already */
if (1 == page_count(cursor_page)) {
	put_page(cursor_page)
	continue;
}
put_page(cursor_page);


> +				 * track the head status without a
> +				 * page pin.
> +				 */
> +				if (!PageTail(cursor_page) &&
> +				    !atomic_read(&cursor_page->_count))
>  					continue;
>  				break;

>  			}

-- 
Kind regards
Minchan Kim
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