Re: [PATCH] mm: Check if any page in a pageblock is reserved beforemarking it MIGRATE_RESERVE

[Mel Gorman]

On Tue, Apr 26, 2011 at 06:49:39PM +0900, KOSAKI Motohiro wrote:
> > On Thu, Apr 21, 2011 at 06:34:03PM -0700, John Stultz wrote:
> > > From: Arve Hjønnevåg <arve@xxxxxxxxxxx>
> > > 
> > > This fixes a problem where the first pageblock got marked MIGRATE_RESERVE even
> > > though it only had a few free pages. This in turn caused no contiguous memory
> > > to be reserved and frequent kswapd wakeups that emptied the caches to get more
> > > contiguous memory.
> > > 
> > > CC: Dave Hansen <dave@xxxxxxxxxxxxxxxxxx>
> > > CC: Mel Gorman <mgorman@xxxxxxx>
> > > CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Arve Hjønnevåg <arve@xxxxxxxxxxx>
> > > Acked-by: Mel Gorman <mel@xxxxxxxxx>
> > > 
> > > [This patch was submitted and acked a little over a year ago
> > > (see: http://lkml.org/lkml/2010/4/6/172 ), but never seemingly
> > > made it upstream. Resending for comments. -jstultz]
> > > 
> > > Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> > 
> > Whoops, should have spotted it slipped through. FWIW, I'm still happy
> > with my Ack being stuck onto it.
> 
> Hehe, No.
> 
> You acked another patch at last year and John taked up old one. Sigh.
> Look,  correct one has pfn_valid_within(). 
> 	http://lkml.org/lkml/2010/4/6/172
> 

Bah, you're right thanks for catching that. A pfn_valid_within check is
indeed required, particularly on ARM where there can be holes punched within
pageblock boundaries. Thanks

> 
> Subject: [PATCH] mm: Check if any page in a pageblock is reserved before marking it MIGRATE_RESERVE
> From: Arve Hjonnevag <arve@xxxxxxxxxxx>
> 
> This fixes a problem where the first pageblock got marked MIGRATE_RESERVE even
> though it only had a few free pages. eg, On current ARM port, The kernel starts
> at offset 0x8000 to leave room for boot parameters, and the memory is freed later.
> 
> This in turn caused no contiguous memory to be reserved and frequent kswapd
> wakeups that emptied the caches to get more contiguous memory.
> 
> Unfortunatelly, ARM need order-2 allocation for pgd (see arm/mm/pgd.c#pgd_alloc()).
> Therefore the issue is not minor nor easy avoidable.
> 
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Arve Hjonnevag <arve@xxxxxxxxxxx>
> Acked-by: Mel Gorman <mel@xxxxxxxxx>
> Acked-by: Dave Hansen <dave@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> [added a
> few explanation]
> ---
>  mm/page_alloc.c |   16 +++++++++++++++-
>  1 files changed, 15 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1d5c189..10d9fa7 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3282,6 +3282,20 @@ static inline unsigned long wait_table_bits(unsigned long size)
>  #define LONG_ALIGN(x) (((x)+(sizeof(long))-1)&~((sizeof(long))-1))
>  
>  /*
> + * Check if a pageblock contains reserved pages
> + */
> +static int pageblock_is_reserved(unsigned long start_pfn)
> +{
> +	unsigned long end_pfn = start_pfn + pageblock_nr_pages;
> +	unsigned long pfn;
> +
> +	for (pfn = start_pfn; pfn < end_pfn; pfn++)
> +		if (!pfn_valid_within(pfn) || PageReserved(pfn_to_page(pfn)))
> +			return 1;
> +	return 0;
> +}
> +
> +/*
>   * Mark a number of pageblocks as MIGRATE_RESERVE. The number
>   * of blocks reserved is based on min_wmark_pages(zone). The memory within
>   * the reserve will tend to store contiguous free pages. Setting min_free_kbytes
> @@ -3320,7 +3334,7 @@ static void setup_zone_migrate_reserve(struct zone *zone)
>  			continue;
>  
>  		/* Blocks with reserved pages will never free, skip them. */
> -		if (PageReserved(page))
> +		if (pageblock_is_reserved(pfn))
>  			continue;
>  
>  		block_migratetype = get_pageblock_migratetype(page);

-- 
Mel Gorman
SUSE Labs
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