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[Xen-devel] [PATCH 10/10] xfs: eagerly remove vmap mappings to avoid ups

To: LKML <linux-kernel@xxxxxxxxxxxxxxx>
Subject: [Xen-devel] [PATCH 10/10] xfs: eagerly remove vmap mappings to avoid upsetting Xen
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 12 Oct 2007 14:11:42 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxx, Mark Williamson <mark.williamson@xxxxxxxxxxxx>, XFS masters <xfs-masters@xxxxxxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>, Andi Kleen <ak@xxxxxx>, Morten Bøgeskov <xen-users@xxxxxxxxxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir@xxxxxxxxxxxxx>, Stable kernel <stable@xxxxxxxxxx>
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XFS leaves stray mappings around when it vmaps memory to make it
virtually contigious.  This upsets Xen if one of those pages is being
recycled into a pagetable, since it finds an extra writable mapping of
the page.

This patch solves the problem in a brute force way, by making XFS
always eagerly unmap its mappings.

[ Stable: This works around a bug in 2.6.23.  We may come up with a
better solution for mainline, but this seems like a low-impact fix for
the stable kernel. ]

Signed-off-by: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>
Cc: XFS masters <xfs-masters@xxxxxxxxxxx>
Cc: Stable kernel <stable@xxxxxxxxxx>
Cc: Morten =?utf-8?q?B=C3=B8geskov?= <xen-users@xxxxxxxxxxxxxxxxxx>
Cc: Mark Williamson <mark.williamson@xxxxxxxxxxxx>

---
 fs/xfs/linux-2.6/xfs_buf.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

===================================================================
--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -186,6 +186,19 @@ free_address(
        void            *addr)
 {
        a_list_t        *aentry;
+
+#ifdef CONFIG_XEN
+       /*
+        * Xen needs to be able to make sure it can get an exclusive
+        * RO mapping of pages it wants to turn into a pagetable.  If
+        * a newly allocated page is also still being vmap()ed by xfs,
+        * it will cause pagetable construction to fail.  This is a
+        * quick workaround to always eagerly unmap pages so that Xen
+        * is happy.
+        */
+       vunmap(addr);
+       return;
+#endif
 
        aentry = kmalloc(sizeof(a_list_t), GFP_NOWAIT);
        if (likely(aentry)) {

-- 


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