Re: ZERO_PAGE refcounting causes cache line bouncing

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

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Nick Piggin wrote:

> I've always thought the bouncing issue was a silly one and should be
> fixed, of course. Maybe the reason my fix was vetoed was lack of numbers.
> Christoph, would you oblige? I'll dig out the patch and repost.

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.

-- 
SUSE Labs, Novell Inc.

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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;
 			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);