[PATCH]: Better checking in range_straddles_page_boundary

[Chris Lalancette <clalance@redhat.com>]

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All,
    Attached is a simple patch to slightly rework the logic in
range_straddles_page_boundary().  The reason for this is to avoid a crash we are
seeing on 32-bit dom0.  Basically, the contiguous_bitmap is allocated based on
max_low_pfn.  However, the swiotlb code can be passed a request (in
swiotlb_map_sg) that is > 1 page (I've seen 2 pages), and is also a page
(sg->page) > max_low_pfn.  If this combination happens, then you get a fatal
page fault when doing the test_bit(pfn, contiguous_bitmap).  For that reason,
rework the logic in range_straddles_page_boundary so that if it gets a request >
1 page, and it's above max_low_pfn, then we force the splitting of the request.
 In our testing, this seems to fix the issue.

Note that the patch is against the RHEL-5 2.6.18 code, but the patch should
apply with a little fuzz to the linux-2.6.18-xen.hg tree.

Signed-off-by: Chris Lalancette <clalance@redhat.com>

[-- Attachment #2: linux-2.6.18-xen-contig_bitmap-more-checking.patch --]
[-- Type: text/x-patch, Size: 1385 bytes --]

--- linux-2.6.18.noarch/arch/i386/kernel/pci-dma-xen.c.orig	2008-10-17 16:58:03.000000000 +0200
+++ linux-2.6.18.noarch/arch/i386/kernel/pci-dma-xen.c	2008-10-17 16:58:34.000000000 +0200
@@ -13,6 +13,7 @@
 #include <linux/pci.h>
 #include <linux/module.h>
 #include <linux/version.h>
+#include <linux/bootmem.h>
 #include <asm/io.h>
 #include <xen/balloon.h>
 #include <asm/swiotlb.h>
@@ -134,12 +135,33 @@ int range_straddles_page_boundary(paddr_
 	unsigned long pfn = p >> PAGE_SHIFT;
 	unsigned int offset = p & ~PAGE_MASK;
 
+	/*
+	 * 1.  If this request is going to stay on this page, it won't
+	 * cross a page boundary
+	 */
 	if (offset + size <= PAGE_SIZE)
 		return 0;
-	if (test_bit(pfn, contiguous_bitmap))
-		return 0;
+	/*
+	 * 2.  If all of the pages here are physically contiguous, we
+	 * technically cross a page boundary, but that's OK
+	 */
 	if (check_pages_physically_contiguous(pfn, offset, size))
 		return 0;
+	/*
+	 * 3.  If the above two tests failed, *and* we are a page > max_low_pfn,
+	 * then we have to straddle a page boundary (we won't be in the
+	 * contiguous_bitmap, and attempting to access it will cause a
+	 * page fault
+	 */
+	if (pfn > max_low_pfn)
+		return 1;
+	/*
+	 * 4.  Finally, check the contiguous bitmap for this pfn; if it's in
+	 * there, we are OK.
+	 */
+	  
+	if (test_bit(pfn, contiguous_bitmap))
+		return 0;
 	return 1;
 }
 

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