Re: [PATCH] mm : add check for the return value

[Minchan Kim <minchan.kim@gmail.com>]

Hi, Huang. 

On Mon,  4 Jan 2010 10:22:10 +0800
Huang Shijie <shijie8@gmail.com> wrote:

> When the `page' returned by __rmqueue() is NULL, the origin code
> still adds -(1 << order) to zone's NR_FREE_PAGES item.
> 
> The patch fixes it.
> 
> Signed-off-by: Huang Shijie <shijie8@gmail.com>
> ---
>  mm/page_alloc.c |   10 +++++++---
>  1 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4e9f5cc..620921d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1222,10 +1222,14 @@ again:
>  		}
>  		spin_lock_irqsave(&zone->lock, flags);
>  		page = __rmqueue(zone, order, migratetype);
> -		__mod_zone_page_state(zone, NR_FREE_PAGES, -(1 << order));
> -		spin_unlock(&zone->lock);
> -		if (!page)
> +		if (likely(page)) {
> +			__mod_zone_page_state(zone, NR_FREE_PAGES,
> +						-(1 << order));
> +			spin_unlock(&zone->lock);
> +		} else {
> +			spin_unlock(&zone->lock);
>  			goto failed;
> +		}
>  	}
>  
>  	__count_zone_vm_events(PGALLOC, zone, 1 << order);

I think it's not desirable to add new branch in hot-path even though
we could avoid that. 

How about this?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4e4b5b3..87976ad 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1244,6 +1244,9 @@ again:
        return page;
 
 failed:
+       spin_lock(&zone->lock);
+       __mod_zone_page_state(zone, NR_FREE_PAGES, 1 << order);
+       spin_unlock(&zone->lock);
        local_irq_restore(flags);
        put_cpu();
        return NULL;




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-- 
Kind regards,
Minchan Kim

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