[Xen-devel] [PATCH]: Better checking in range_straddles_page_boundary

[Chris Lalancette <clalance@xxxxxxxxxx>]

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@xxxxxxxxxx>

--- 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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