Re: [PATCH -v2] mm: more likely reclaim MADV_SEQUENTIAL mappings

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 6 Nov 2008 10:55:13 +0100
Johannes Weiner <hannes@cmpxchg.org> wrote:

> File pages mapped only in sequentially read mappings are perfect
> reclaim canditates.
> 
> This patch makes these mappings behave like weak references, their
> pages will be reclaimed unless they have a strong reference from a
> normal mapping as well.
> 
> It changes the reclaim and the unmap path where they check if the page
> has been referenced.  In both cases, accesses through sequentially
> read mappings will be ignored.
> 
> Signed-off-by: Johannes Weiner <hannes@saeurebad.de>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> CC: Nick Piggin <npiggin@suse.de>
> ---
> 
> II: add likely()s to mitigate the extra branches a bit as to Nick's
>     suggestion
> 
> Benchmark results from KOSAKI Motohiro:
> 
>     http://marc.info/?l=linux-mm&m=122485301925098&w=2
> 
>  mm/memory.c |    3 ++-
>  mm/rmap.c   |   13 +++++++++++--
>  2 files changed, 13 insertions(+), 3 deletions(-)
> 
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -337,8 +337,17 @@ static int page_referenced_one(struct pa
>  		goto out_unmap;
>  	}
>  
> -	if (ptep_clear_flush_young_notify(vma, address, pte))
> -		referenced++;
> +	if (ptep_clear_flush_young_notify(vma, address, pte)) {
> +		/*
> +		 * Don't treat a reference through a sequentially read
> +		 * mapping as such.  If the page has been used in
> +		 * another mapping, we will catch it; if this other
> +		 * mapping is already gone, the unmap path will have
> +		 * set PG_referenced or activated the page.
> +		 */
> +		if (likely(!VM_SequentialReadHint(vma)))
> +			referenced++;
> +	}
>  
>  	/* Pretend the page is referenced if the task has the
>  	   swap token and is in the middle of a page fault. */
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -759,7 +759,8 @@ static unsigned long zap_pte_range(struc
>  			else {
>  				if (pte_dirty(ptent))
>  					set_page_dirty(page);
> -				if (pte_young(ptent))
> +				if (pte_young(ptent) &&
> +				    likely(!VM_SequentialReadHint(vma)))
>  					mark_page_accessed(page);
>  				file_rss--;
>  			}

Of course, in the majority of cases those applications which are
performing sequential reads will have forgotten to call
madvise(MADV_SEQUENTIAL).

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