Re: ZERO_PAGE refcounting causes cache line bouncing

[Nick Piggin <nickpiggin@yahoo.com.au>]

Nick Piggin wrote:

> Something like this roughly should get rid of ZERO_PAGE _count and 
> _mapcount
> manipulation for anonymous pages. (others still exist, XIP and 
> /dev/zero, but
> they should not be a large concern AFAIKS).
> 
> I haven't booted this, but it is a quick forward port + some fixes and
> simplifications.
> 
> 
> ------------------------------------------------------------------------
> 
> Index: linux-2.6/mm/memory.c
> ===================================================================
> --- linux-2.6.orig/mm/memory.c
> +++ linux-2.6/mm/memory.c
> @@ -665,7 +665,8 @@ static unsigned long zap_pte_range(struc
>  			ptent = ptep_get_and_clear_full(mm, addr, pte,
>  							tlb->fullmm);
>  			tlb_remove_tlb_entry(tlb, pte, addr);
> -			if (unlikely(!page))
> +			if (unlikely(!page ||
> +				(!vma->vm_file && page == ZERO_PAGE(addr))))
>  				continue;

Hmm, well I suppose it would be cleaner if this check used the one in
handle_pte_fault instead of !vma->vm_file ie. (!vma->vm_ops ||
!vma->vm_ops->nopage)


>  			if (unlikely(details) && details->nonlinear_vma
>  			    && linear_page_index(details->nonlinear_vma,
> @@ -2152,15 +2153,12 @@ static int do_anonymous_page(struct mm_s
>  	} else {
>  		/* Map the ZERO_PAGE - vm_page_prot is readonly */
>  		page = ZERO_PAGE(address);
> -		page_cache_get(page);
>  		entry = mk_pte(page, vma->vm_page_prot);
>  
>  		ptl = pte_lockptr(mm, pmd);
>  		spin_lock(ptl);
>  		if (!pte_none(*page_table))
> -			goto release;
> -		inc_mm_counter(mm, file_rss);
> -		page_add_file_rmap(page);
> +			goto unlock;
>  	}
>  
>  	set_pte_at(mm, address, page_table, entry);


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