PATCH Migration: find correct vma in new_vma_page()

PATCH Migration: find correct vma in new_vma_page()

Against: 2.6.23-mm1

We hit the BUG_ON() in mm/rmap.c:vma_address() when trying to migrate via mbind(MPOL_MF_MOVE) a non-anon region that spans multiple vmas. For anon-regions, we just fail to migrate any pages beyond the 1st vma in the range.

This occurs because do_mbind() collects a list of pages to migrate by calling check_range(). check_range() walks the task's mm, spanning vmas as necessary, to collect the migratable pages into a list. Then, do_mbind() calls migrate_pages() passing the list of pages, a function to allocate new pages based on vma policy [new_vma_page()], and a pointer to the first vma of the range.

For each page in the list, new_vma_page() calls page_address_in_vma() passing the page and the vma [first in range] to obtain the address to get for alloc_page_vma(). The page address is needed to get interleaving policy correct. If the pages in the list come from multiple vmas, eventually, new_page_address() will pass that page to page_address_in_vma() with the incorrect vma. For !PageAnon pages, this will result in a bug check in rmap.c:vma_address(). For anon pages, vma_address() will just return EFAULT and fail the migration.

This patch modifies new_vma_page() to check the return value from page_address_in_vma(). If the return value is EFAULT, new_vma_page() searchs forward via vm_next for the vma that maps the page--i.e., that does not return EFAULT. This assumes that the pages in the list handed to migrate_pages() is in address order. This is currently case. The patch documents this assumption in a new comment block for new_vma_page().

If new_vma_page() cannot locate the vma mapping the page in a forward search in the mm, it will pass a NULL vma to alloc_page_vma(). This will result in the allocation using the task policy, if any, else system default policy. This situation is unlikely, but the patch documents this behavior with a comment.

Note, this patch results in restarting from the first vma in a multi-vma range each time new_vma_page() is called. If this is not acceptable, we can make the vma argument a pointer, both in new_vma_page() and it's caller unmap_and_move() so that the value held by the loop in migrate_pages() always passes down the last vma in which a page was found. This will require changes to all new_page_t functions passed to migrate_pages(). Is this necessary?

For this patch to work, we can't bug check in vma_address() for pages outside the argument vma. This patch removes the BUG_ON(). All other callers [besides new_vma_page()] already check the return status.

Tested on x86_64, 4 node NUMA platform.

 mm/mempolicy.c |   21 +++++++++++++++++++--
 mm/rmap.c      |    7 ++++---
 2 files changed, 23 insertions(+), 5 deletions(-)

Index: Linux/mm/mempolicy.c
===================================================================
@@ -722,12 +722,29 @@ out:
 
 }
 
+/*
+ * Allocate a new page for page migration based on vma policy.
+ * Start assuming that page is mapped by vma pointed to by @private.
+ * Search forward from there, if not.  N.B., this assumes that the
+ * list of pages handed to migrate_pages()--which is how we get here--
+ * is in virtual address order.
+ */
 static struct page *new_vma_page(struct page *page, unsigned long private, int **x)
 {
 	struct vm_area_struct *vma = (struct vm_area_struct *)private;
+	unsigned long uninitialized_var(address);
 
-	return alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma,
-					page_address_in_vma(page, vma));
+	while (vma) {
+		address = page_address_in_vma(page, vma);
+		if (address != -EFAULT)
+			break;
+		vma = vma->vm_next;
+	}
+
+	/*
+	 * if !vma, alloc_page_vma() will use task or system default policy
+	 */
+	return alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
 }
 #else
 

Index: Linux/mm/rmap.c
===================================================================
@@ -184,7 +184,9 @@ static void page_unlock_anon_vma(struct 
 }
 
 /*
- * At what user virtual address is page expected in vma?
+ * At what user virtual address is page expected in @vma?
+ * Returns virtual address or -EFAULT if page's index/offset is not
+ * within the range mapped the @vma.
  */
 static inline unsigned long
 vma_address(struct page *page, struct vm_area_struct *vma)

@@ -194,8 +196,7 @@ vma_address(struct page *page, struct vm
 
 	address = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
 	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
-		/* page should be within any vma from prio_tree_next */
-		BUG_ON(!PageAnon(page));
+		/* page should be within @vma mapping range */
 		return -EFAULT;
 	}
 	return address;

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Looks great! Thanks.

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