From 281ff33b7c1b1ba2a5f9b03425e5f692a94913fa Mon Sep 17 00:00:00 2001 From: Suresh Siddha Date: Thu, 18 Feb 2010 11:51:40 -0800 Subject: [PATCH] x86_64, cpa: Don't work hard in preserving kernel 2M mappings when using 4K already We currently enforce the !RW mapping for the kernel mapping that maps holes between different text, rodata and data sections. However, kernel identity mappings will have different RWX permissions to the pages mapping to text and to the pages padding (which are freed) the text, rodata sections. Hence kernel identity mappings will be broken to smaller pages. For 64-bit, kernel text and kernel identity mappings are different, so we can enable protection checks that come with CONFIG_DEBUG_RODATA, as well as retain 2MB large page mappings for kernel text. Konrad reported a boot failure with the Linux Xen paravirt guest because of this. In this paravirt guest case, the kernel text mapping and the kernel identity mapping share the same page-table pages. Thus forcing the !RW mapping for some of the kernel mappings also cause the kernel identity mappings to be read-only resulting in the boot failure. Linux Xen paravirt guest also uses 4k mappings and don't use 2M mapping. Fix this issue and retain large page performance advantage for native kernels by not working hard and not enforcing !RW for the kernel text mapping, if the current mapping is already using small page mapping. Reported-by: Konrad Rzeszutek Wilk Signed-off-by: Suresh Siddha LKML-Reference: <1266522700.2909.34.camel@sbs-t61.sc.intel.com> Tested-by: Konrad Rzeszutek Wilk Cc: stable@kernel.org [2.6.32, 2.6.33] Signed-off-by: H. Peter Anvin --- arch/x86/mm/pageattr.c | 25 +++++++++++++++++++++++-- 1 file changed, 23 insertions(+), 2 deletions(-) diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c index 1d4eb93d333c..cf07c26d9a4a 100644 --- a/arch/x86/mm/pageattr.c +++ b/arch/x86/mm/pageattr.c @@ -291,8 +291,29 @@ 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)) { + unsigned int level; + + /* + * Don't enforce the !RW mapping for the kernel text mapping, + * if the current mapping is already using small page mapping. + * No need to work hard to preserve large page mappings in this + * case. + * + * This also fixes the Linux Xen paravirt guest boot failure + * (because of unexpected read-only mappings for kernel identity + * mappings). In this paravirt guest case, the kernel text + * mapping and the kernel identity mapping share the same + * page-table pages. Thus we can't really use different + * protections for the kernel text and identity mappings. Also, + * these shared mappings are made of small page mappings. + * Thus this don't enforce !RW mapping for small page kernel + * text mapping logic will help Linux Xen parvirt guest boot + * aswell. + */ + if (lookup_address(address, &level) && (level != PG_LEVEL_4K)) + pgprot_val(forbidden) |= _PAGE_RW; + } #endif prot = __pgprot(pgprot_val(prot) & ~pgprot_val(forbidden));