On Wednesday 05 March 2008 00:47:34 Fabio Checconi wrote:
> > From: Rusty Russell <[EMAIL PROTECTED]>
> > /* Now we reserve the "virtual memory area" we want: 0xFFC00000
> > * (SWITCHER_ADDR). We might not get it in theory, but in practice
> > - * it's worked so far. */
> > + * it's worked so far. The +1 is because __get_vm_area allocates a
> > + * guard page, so we need space for that. */
> > switcher_vma = __get_vm_area(TOTAL_SWITCHER_PAGES * PAGE_SIZE,
> > - VM_ALLOC, SWITCHER_ADDR, VMALLOC_END);
> > + VM_ALLOC, SWITCHER_ADDR, SWITCHER_ADDR
> > + + (TOTAL_SWITCHER_PAGES+1) * PAGE_SIZE);
> > if (!switcher_vma) {
> > err = -ENOMEM;
> > printk("lguest: could not map switcher pages high\n");
>
> With the original .config, VMALLOC_END was lower than SWITCHER_ADDR,
> with this patch we may be forcing an allocation beyond VMALLOC_END,
> and this may cause the allocated region to fall in the fixmap/PKMAP
> area.
Hi Fabio,
Yes, it's true we can allocate beyond VMALLOC_END, but that's OK: we're not
a vmalloc. We currently will not be in the fixmap area by a long way, but
I've added a check, just in case.
Also, the previous comment confused inflo: is this one better?
Thanks for the feedback!
Rusty.
==
Robert Bragg's 5dc331852848a38ca00a2817e5b98a1d0561b116 tightened
(ie. fixed) the checking in __get_vm_area, and it broke lguest.
lguest should pass the exact "end" it wants, not some random constant
(it was possible previously that it would actually get an address
different from SWITCHER_ADDR).
Also, Fabio Checconi pointed out that we should make sure we're not
hitting the fixmap area.
Signed-off-by: Rusty Russell <[EMAIL PROTECTED]>
Cc: Robert Bragg <[EMAIL PROTECTED]>
---
drivers/lguest/core.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff -r f5dc1442dc61 drivers/lguest/core.c
--- a/drivers/lguest/core.c Tue Mar 04 14:21:38 2008 +1100
+++ b/drivers/lguest/core.c Wed Mar 05 14:16:43 2008 +1100
@@ -69,11 +69,22 @@ static __init int map_switcher(void)
switcher_page[i] = virt_to_page(addr);
}
+ /* First we check that the Switcher won't overlap the fixmap area at
+ * the top of memory. It's currently nowhere near, but it could have
+ * very strange effects if it ever happened. */
+ if (SWITCHER_ADDR + (TOTAL_SWITCHER_PAGES+1)*PAGE_SIZE > FIXADDR_START){
+ err = -ENOMEM;
+ printk("lguest: mapping switcher would thwack fixmap\n");
+ goto free_pages;
+ }
+
/* Now we reserve the "virtual memory area" we want: 0xFFC00000
* (SWITCHER_ADDR). We might not get it in theory, but in practice
- * it's worked so far. */
+ * it's worked so far. The end address needs +1 because __get_vm_area
+ * allocates an extra guard page, so we need space for that. */
switcher_vma = __get_vm_area(TOTAL_SWITCHER_PAGES * PAGE_SIZE,
- VM_ALLOC, SWITCHER_ADDR, VMALLOC_END);
+ VM_ALLOC, SWITCHER_ADDR, SWITCHER_ADDR
+ + (TOTAL_SWITCHER_PAGES+1) * PAGE_SIZE);
if (!switcher_vma) {
err = -ENOMEM;
printk("lguest: could not map switcher pages high\n");
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