From: Henry Nestler Date: Mon, 12 May 2008 13:44:39 +0000 (+0200) Subject: x86: fix endless page faults in mount_block_root for Linux 2.6 X-Git-Tag: v2.6.26-rc6~1^2~7 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b29c701deacd5d24453127c37ed77ef851c53b8b;p=linux-2.6 x86: fix endless page faults in mount_block_root for Linux 2.6 Page faults in kernel address space between PAGE_OFFSET up to VMALLOC_START should not try to map as vmalloc. Fix rarely endless page faults inside mount_block_root for root filesystem at boot time. All 32bit kernels up to 2.6.25 can fail into this hole. I can not present this under native linux kernel. I see, that the 64bit has fixed the problem. I copied the same lines into 32bit part. Recorded debugs are from coLinux kernel 2.6.22.18 (virtualisation): http://www.henrynestler.com/colinux/testing/pfn-check-0.7.3/20080410-antinx/bug16-recursive-page-fault-endless.txt The physicaly memory was trimmed down to 192MB to better catch the bug. More memory gets the bug more rarely. Details, how every x86 32bit system can fail: Start from "mount_block_root", http://lxr.linux.no/linux/init/do_mounts.c#L297 There the variable "fs_names" got one memory page with 4096 bytes. Variable "p" walks through the existing file system types. The first string is no problem. But, with the second loop in mount_block_root the offset of "p" is not at beginning of page, the offset is for example +9, if "reiserfs" is the first in list. Than calls do_mount_root, and lands in sys_mount. Remember: Variable "type_page" contains now "fs_type+9" and not contains a full page. The sys_mount copies 4096 bytes with function "exact_copy_from_user()": http://lxr.linux.no/linux/fs/namespace.c#L1540 Mostly exist pages after the buffer "fs_names+4096+9" and the page fault handler was not called. No problem. In the case, if the page after "fs_names+4096" is not mapped, the page fault handler was called from http://lxr.linux.no/linux/fs/namespace.c#L1320 The do_page_fault gots an address 0xc03b4000. It's kernel address, address >= TASK_SIZE, but not from vmalloc! It's from "__getname()" alias "kmem_cache_alloc". The "error_code" is 0. "vmalloc_fault" will be call: http://lxr.linux.no/linux/arch/i386/mm/fault.c#L332 "vmalloc_fault" tryed to find the physical page for a non existing virtual memory area. The macro "pte_present" in vmalloc_fault() got a next page fault for 0xc0000ed0 at: http://lxr.linux.no/linux/arch/i386/mm/fault.c#L282 No PTE exist for such virtual address. The page fault handler was trying to sync the physical page for the PTE lockup. This called vmalloc_fault() again for address 0xc000000, and that also was not existing. The endless began... In normal case the cpu would still loop with disabled interrrupts. Under coLinux this was catched by a stack overflow inside printk debugs. Signed-off-by: Henry Nestler Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner --- diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index fd7e1798c7..8bcb6f40cc 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -497,6 +497,11 @@ static int vmalloc_fault(unsigned long address) unsigned long pgd_paddr; pmd_t *pmd_k; pte_t *pte_k; + + /* Make sure we are in vmalloc area */ + if (!(address >= VMALLOC_START && address < VMALLOC_END)) + return -1; + /* * Synchronize this task's top level page-table * with the 'reference' page table.