Re: nonlinear swapping w/o pte_chains [Re: VMA_MERGING_FIXUP and patch]

[Andrea Arcangeli]

On Wed, Mar 24, 2004 at 03:37:29PM +0100, Andrea Arcangeli wrote:
> cute, I agree we should recheck the young bit inside.

I did it (the below is incremental with 2.6.5-rc2-aa2), looks ok but I
cannot notice absolutely any difference in practice. Anyways it makes
sense so I keep it. now re-running the regressions for both linear and
non-linear heavy shm swap.

So I'm now going to merge the prio_tree, then I can care about the
filter for s/anon_vma_t/struct anon_vma/ and the mprotect/mremap
vma merging (for file mappings too).

--- x/mm/objrmap.c.~1~	2004-03-24 07:16:18.000000000 +0100
+++ x/mm/objrmap.c	2004-03-24 19:27:31.213416608 +0100
@@ -137,7 +137,7 @@ page_referenced_one(struct vm_area_struc
 
 	pte = find_pte(vma, page, NULL);
 	if (pte) {
-		if (ptep_test_and_clear_young(pte))
+		if (pte_young(*pte) && ptep_test_and_clear_young(pte))
 			referenced++;
 		pte_unmap(pte);
 	}
@@ -442,6 +442,8 @@ try_to_unmap_nonlinear_pte(struct vm_are
 			page = pfn_to_page(pfn);
 			if (PageReserved(page))
 				continue;
+			if (pte_young(pte) && ptep_test_and_clear_young(ptep))
+				continue;
 			/*
 			 * any other page in the nonlinear mapping will not wait
 			 * on us since only one cpu can take the i_shared_sem
@@ -506,7 +508,7 @@ try_to_unmap_nonlinear(struct vm_area_st
  * This function is strictly a helper function for try_to_unmap_inode.
  */
 static int
-try_to_unmap_one(struct vm_area_struct *vma, struct page *page)
+try_to_unmap_one(struct vm_area_struct *vma, struct page *page, int * young)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	unsigned long address;
@@ -523,12 +525,21 @@ try_to_unmap_one(struct vm_area_struct *
 
 	if (unlikely(vma->vm_flags & VM_NONLINEAR)) {
 		/*
-		 * All it matters is that the page won't go
-		 * away under us after we unlock.
+		 * If this was a false positive generated by a
+		 * failed trylock in the referenced pass let's
+		 * avoid to pay the big cost of the nonlinear
+		 * swap, we'd better be sure we've to pay that
+		 * cost before running it.
 		 */
-		page_map_unlock(page);
-		try_to_unmap_nonlinear(vma);
-		page_map_lock(page);
+		if (!*young) {
+			/*
+			 * All it matters is that the page won't go
+			 * away under us after we unlock.
+			 */
+			page_map_unlock(page);
+			try_to_unmap_nonlinear(vma);
+			page_map_lock(page);
+		}
 		goto out;
 	}
 
@@ -536,10 +547,21 @@ try_to_unmap_one(struct vm_area_struct *
 	if (!pte)
 		goto out;
 
-	unmap_pte_page(page, vma, address, pte);
+	/*
+	 * We use trylocks in the "reference" methods, if they fails
+	 * we let the VM to go ahead unmapping to avoid locking
+	 * congestions, so here we may be trying to unmap young
+	 * ptes, if that happens we givup trying unmapping this page
+	 * and we clear all other reference bits instead (basically
+	 * downgrading to a page_referenced pass).
+	 */
+	if ((!pte_young(*pte) || !ptep_test_and_clear_young(pte)) && !*young)
+		unmap_pte_page(page, vma, address, pte);
+	else
+		*young = 1;
 
 	pte_unmap(pte);
-out:
+ out:
 	spin_unlock(&mm->page_table_lock);
 	return ret;
 }
@@ -561,7 +583,7 @@ try_to_unmap_inode(struct page *page)
 {
 	struct address_space *mapping = page->mapping;
 	struct vm_area_struct *vma;
-	int ret = SWAP_AGAIN;
+	int ret = SWAP_AGAIN, young = 0;
 
 	BUG_ON(PageSwapCache(page));
 
@@ -569,13 +591,13 @@ try_to_unmap_inode(struct page *page)
 		return ret;
 	
 	list_for_each_entry(vma, &mapping->i_mmap, shared) {
-		ret = try_to_unmap_one(vma, page);
+		ret = try_to_unmap_one(vma, page, &young);
 		if (ret == SWAP_FAIL || !page->mapcount)
 			goto out;
 	}
 
 	list_for_each_entry(vma, &mapping->i_mmap_shared, shared) {
-		ret = try_to_unmap_one(vma, page);
+		ret = try_to_unmap_one(vma, page, &young);
 		if (ret == SWAP_FAIL || !page->mapcount)
 			goto out;
 	}
@@ -588,7 +610,7 @@ out:
 static int
 try_to_unmap_anon(struct page * page)
 {
-	int ret = SWAP_AGAIN;
+	int ret = SWAP_AGAIN, young = 0;
 	struct vm_area_struct * vma;
 	anon_vma_t * anon_vma = (anon_vma_t *) page->mapping;
 
@@ -598,7 +620,7 @@ try_to_unmap_anon(struct page * page)
 	spin_lock(&anon_vma->anon_vma_lock);
 	BUG_ON(list_empty(&anon_vma->anon_vma_head));
 	list_for_each_entry(vma, &anon_vma->anon_vma_head, anon_vma_node) {
-		ret = try_to_unmap_one(vma, page);
+		ret = try_to_unmap_one(vma, page, &young);
 		if (ret == SWAP_FAIL || !page->mapcount)
 			break;
 	}

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