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From: Konrad Rzeszutek Wilk on 12 Feb 2010 22:40
This fixes BUG: unable to handle kernel paging request at ffff8800013f4000

IP: [<ffffffff8102bb0b>] free_init_pages+0xa3/0xcc
PGD 1611067 PUD 1615067 PMD 556b067 PTE 100000013f4025
....
[<ffffffff8102c2cb>] mark_rodata_ro+0x4a2/0x527
[<ffffffff810021a5>] init_post+0x2b/0x10e
....

On platforms where the pages to be recycled ("free") are 4KB (Xen PV guest).

In the mark_rodata_ro, we set the .text through .sdata section to RO,
then for selective sections (.__stop___ex_table -> .__start_rodata)
and (.__end_rodata -> ._sdata) set them to RW. The logic in
static_protections forbids this and unsets the _PAGE_RW attribute.

This is not an issue if the sections to be recycled are in 2MB pages
so on native platform this isn't seen.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk(a)oracle.com>
---
arch/x86/mm/pageattr.c | 13 +++++++++++--
1 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 1d4eb93..44ebcd7 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -291,8 +291,17 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
*/
if (kernel_set_to_readonly &&
within(address, (unsigned long)_text,
- (unsigned long)__end_rodata_hpage_align))
- pgprot_val(forbidden) |= _PAGE_RW;
+ (unsigned long)__end_rodata_hpage_align)) {
+ /* When 'kernel_set_to_readonly', it is OK to set PAGE_RW
+ * on the ones that are being recycled by free_init_pages
+ * in mark_rodata_ro.
+ */
+ if (!within(address, (unsigned long)&__stop___ex_table,
+ (unsigned long)&__start_rodata) &&
+ !within(address, (unsigned long)&__end_rodata,
+ (unsigned long)&_sdata))
+ pgprot_val(forbidden) |= _PAGE_RW;
+ }
#endif

prot = __pgprot(pgprot_val(prot) & ~pgprot_val(forbidden));
--
1.6.2.5

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Prev: [PATCH 1/6] sysfs: Serialize updates to the vfs inode
Next: tip related: radix tree for spareseirq and logical flat clean up