From 8d9a784d1e2c75e0dcae06f77a02f5e7bb547f3a Mon Sep 17 00:00:00 2001 From: Stuart Menefy Date: Tue, 14 Feb 2012 11:29:11 +0000 Subject: sh: Fix error synchronising kernel page tables The problem is caused by the interaction of two features in the Linux memory management code. A processes address space is described by a struct mm_struct, and every thread has a pointer to the mm it should run in. The exception to this are kernel threads, which don't have an mm, and so borrow the mm from the last thread which ran. The system is bootstrapped by the initial kernel thread using init's mm (even though init hasn't been created yet, its mm is the static init_mm). The other feature is how the kernel handles the page table which describes the portion of the address space which is only visible when executing inside the kernel, and which is shared by all threads. On the SH4 the only portion of the kernel's address space which described using the page table is called P3, from 0xc0000000 to 0xdfffffff. This portion of the address space is divided into three: - mappings for dma_alloc_coherent() - mappings for vmalloc() and ioremap() - fixmap mappings, primarily used in copy_user_pages() to create kernel mappings of user pages with the correct cache colour. To optimise the TLB miss handler we don't want to add an additional condition which checks whether the faulting address is in the user or the kernel portion of the address space, and so all page tables have a common portion which describes the kernel part of the address space. As the SH4 uses a two level page table, only the kernel portion of first level page table (the pgd entries) is duplicated. These all point to the same second level entries (the pte's), and so no memory is wasted. The reference page table for the kernel is called the swapper_pg_dir, and when a new page table is created for a new process the kernel portion of the page table is copied from swapper_pg_dir. This works fine when changes only occur in the second level of the kernel's page table, or the first level entries are created before any new user processes. However if a change occurs to the first level of the page table, and there are existing processes which don't have this entry in their page table, this new entry needs to be added. This is done on demand, when the kernel accesses a P3 address which isn't mapped using the current page table, the code in vmalloc_fault() copies the entry from the reference page table (swapper_pg_dir) into the current processes page table. The bug which this patch addresses is that the code in vmalloc_fault() was not copying addresses which fell in the dma_alloc_coherent() portion of the address space, and it should have been copying any P3 address. Why we hadn't seen this before, and what made this hard to reproduce, is that normally the kernel will have called dma_alloc_coherent(), and accessed the memory mapping created, before any user process runs. Typically drivers such as USB or SATA will have created and used mappings of this type during the kernel initialisation, when probing for the attached devices, before init runs. Ethernet is slightly different, as it normally only creates and accesses dma_alloc_coherent() mappings when the network is brought up, but if kernel level IP configuration is used this will also occur before any user space process runs. So the first reproduction of this problem which we saw was occurred when USB and SATA were removed from the kernel, and then bring up Ethernet from user space using ifconfig. I'd like to thank Joseph Bormolini who did the hard work reducing the problem to this simple to reproduce criteria. In your case the situation is slightly different, and turns out to depends on the exact kernel configuration (which we had) and your ramdisk contents (which we didn't - hence the need for some assumptions). In this case the problem is a side effect of kernel level module loading. Kernel subsystems sometimes trigger the load of kernel modules directly, for example the crypto subsystem tries to load the cryptomgr and MTD tries to load modules for Flash partitioning if these are not built into the kernel. This is done by the kernel creating a user process which runs insmod to try and load the appropriate module. In order for this to cause problems the system must be running with a initrd or initramfs, which contains an insmod executable - if the kernel can't find an insmod to run, no user process is created, and the problem doesn't occur. If an insmod is found, a process is created to run it, which will inherit the kernel portion of the swapper_pg_dir first level page table. It doesn't matter whether the inmod is successful or not, but when the the kernel scheduler context switches back to the kernel initialisation thread, the insmod's mm is 'borrowed' by the kernel thread, as it doesn't have an address space of its own. (Reference counting is used to ensure this mm is not destroyed, even though the user process which caused its creation may no longer exist.) If this address space doesn't have a first level page table entry for the consistent mappings, and a driver tries to access such a mapping, we are in the same situation as described above, except this time in a kernel thread rather than a user thread executing inside the kernel. See bugzilla: 15425, 15836, 15862, 16106, 16793 Signed-off-by: Stuart Menefy Signed-off-by: Paul Mundt diff --git a/arch/sh/mm/fault_32.c b/arch/sh/mm/fault_32.c index 324eef9..e99b104 100644 --- a/arch/sh/mm/fault_32.c +++ b/arch/sh/mm/fault_32.c @@ -86,7 +86,7 @@ static noinline int vmalloc_fault(unsigned long address) pte_t *pte_k; /* Make sure we are in vmalloc/module/P3 area: */ - if (!(address >= VMALLOC_START && address < P3_ADDR_MAX)) + if (!(address >= P3SEG && address < P3_ADDR_MAX)) return -1; /* -- cgit v0.10.2