[PATCH]: Better checking in range_straddles_page_boundary

[PATCH]: Better checking in range_straddles_page_boundary

[Chris Lalancette]

[-- Attachment #1: Type: text/plain, Size: 921 bytes --]

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;
 }
 

[-- Attachment #3: Type: text/plain, Size: 138 bytes --]

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Re: [PATCH]: Better checking in range_straddles_page_boundary

[Keir Fraser]

On 22/10/08 10:05, "Chris Lalancette" <clalance@redhat.com> wrote:

>     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.

How about we just get rid of the contiguous_bitmap? We might as well if you
are going to push that check after calling
check_pages_physically_contiguous(), since no extent which is acceptable to
contiguous_bitmap should fail on check_pages_physically_contiguous(). I
think I kept contiguous_bitmap only as a fast check before calling that
slower function.

 -- Keir

Re: [PATCH]: Better checking in range_straddles_page_boundary

[Chris Lalancette]

Keir Fraser wrote:
> On 22/10/08 10:05, "Chris Lalancette" <clalance@redhat.com> wrote:
> 
>>     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.
> 
> How about we just get rid of the contiguous_bitmap? We might as well if you
> are going to push that check after calling
> check_pages_physically_contiguous(), since no extent which is acceptable to
> contiguous_bitmap should fail on check_pages_physically_contiguous(). I
> think I kept contiguous_bitmap only as a fast check before calling that
> slower function.

Oh, hm, good point.  If we think that the fast check is still good to have, the
other option is to leave range_straddles_page_boundary as-is, and just allocate
contiguous_bitmap with max_pfn (or end_pfn; I can never remember which is which
i386 vs. x86_64).

Either way works for me, though; I don't think the
check_pages_physically_contiguous is a huge performance bottleneck, especially
since *most* requests are for 1 page; it's just the odd request that comes in
with 2 pages (or more, but I've never seen more than 2 pages).

-- 
Chris Lalancette