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[Xen-devel] Vanilla Linux and has_foreign_mapping

To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Vanilla Linux and has_foreign_mapping
From: Michael Abd-El-Malek <mabdelmalek@xxxxxxx>
Date: Sun, 20 Apr 2008 17:19:20 -0400
Cc: Mark McLoughlin <markmc@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Eduardo Habkost <ehabkost@xxxxxxxxxx>
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I'm trying to add support to Linux 2.6.25 for the "has_foreign_mappings" MMU context flag. Xen's Linux 2.6.18 tree uses this flag, so that page tables are properly disposed of when an application exits when it has foreign mappings. See:

Here is my attempt:
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 2a054ef..3e51897 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -633,8 +633,13 @@ void xen_exit_mmap(struct mm_struct *mm)

        /* pgd may not be pinned in the error exit path of execve */
-       if (PagePinned(virt_to_page(mm->pgd)))
-               xen_pgd_unpin(mm->pgd);
+       if (PagePinned(virt_to_page(mm->pgd))) {
+        if (mm->context.has_foreign_mappings) {
+ printk("%s: because of has_foreign_mappings, delaying unpinning\n", __FUNCTION__);
+        } else {
+            xen_pgd_unpin(mm->pgd);
+        }
+    }

diff --git a/include/asm-x86/mmu.h b/include/asm-x86/mmu.h
index efa962c..7194698 100644
--- a/include/asm-x86/mmu.h
+++ b/include/asm-x86/mmu.h
@@ -18,6 +18,9 @@ typedef struct {
        int size;
        struct mutex lock;
        void *vdso;
+#ifdef CONFIG_XEN
+       int has_foreign_mappings;
 } mm_context_t;

 #ifdef CONFIG_SMP

Unfortunately, I got the following kernel crash on process exit:

BUG: unable to handle kernel paging request at ebdae008
IP: [<c01157f9>] pgd_mop_up_pmds+0x6a/0xd8
*pdpt = 000000007f494027
Oops: 0003 [#1] PREEMPT SMP
Modules linked in: efsvm(F) nfs lockd sunrpc dm_snapshot dm_mirror dm_mod

Pid: 5565, comm: a.out Tainted: GF        (2.6.25 #9)
EIP: 0061:[<c01157f9>] EFLAGS: 00010246 CPU: 0
EIP is at pgd_mop_up_pmds+0x6a/0xd8
Call Trace:
 [<c01158bf>] pgd_free+0x8/0x19
 [<c011fca0>] __mmdrop+0x16/0x2a
 [<c01244bc>] do_exit+0x1b3/0x569
 [<c01248d5>] do_group_exit+0x63/0x7a
 [<c0107066>] syscall_call+0x7/0xb

Has anyone else implemented this functionality in the mainline Linux tree? Any thoughts?


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