mapping contiguous kernel memory to user space
 

Hello,

I'm using linux kernel 2.6.29.1 I've been using the mmap system call to map kernel memory (RAM), obtained by get_free_pages, to user space. My driver uses the "fault" (previously called nopage) method for that purpose.

here's a snipet of the fault callback;

offset = vmf->pgoff << PAGE_SHIFT;
if( offset > dev->dma_buff_size)
{
printk("ds3b3_vm_fault: SIGBUS - my_offset: %#x vmf_pgoff: %#x page_shift: %i \n",
offset, vmf->pgoff, PAGE_SHIFT);
return VM_FAULT_SIGBUS;
}

addr = (char *)vma->vm_start;
addr += offset;

page = virt_to_page( addr );

get_page(page);
vmf->page = page;

When the application loads the module the following is printed on the console;

<4>ds3b3_vm_fault: entered - vma->vm_start: 0x48000000 vma->vm_end 0x483fd000
<4>ds3b3_vm_fault: entered - vma->vm_flags: 0x820fb vma->vm_pgoff 0x0
<4>ds3b3_vm_fault: entered - vmf->flags: 0x1 vmf->pgoff 0x0 vmf->virtual_address: 0x48000000
<4>ds3b3_vm_fault: SUCCESS - vmf->page: 0xc13d1000
<1>BUG: Bad page map in process dcb pte:880004d2 pmd:0c5ec400
<1>addr:48000000 vm_flags:000820fb anon_vma:(null) mapping:ce4927d0 index:0
<1>vma->vm_ops->fault: ds3b3_vm_fault+0x0/0xf8 [ds3b3]
<1>vma->vm_file->f_op->mmap: ds3b3_nopage_mmap+0x0/0x4c [ds3b3]
<4>Call Trace:
<4>[cd659d80] [c0006bc0] show_stack+0x44/0x16c (unreliable)
<4>[cd659dc0] [c00627c8] print_bad_pte+0x140/0x1cc
<4>[cd659df0] [c00628d0] vm_normal_page+0x7c/0xb4
<4>[cd659e00] [c00630b4] follow_page+0xf4/0x1f0
<4>[cd659e20] [c00645e4] __get_user_pages+0x130/0x3ec
<4>[cd659e80] [c0064b28] make_pages_present+0x8c/0xc4
<4>[cd659e90] [c0066780] mlock_vma_pages_range+0x74/0x9c
<4>[cd659eb0] [c0068efc] mmap_region+0x1dc/0x3c8
<4>[cd659f10] [c00037b8] sys_mmap+0x78/0x100
<4>[cd659f40] [c000e558] ret_from_syscall+0x0/0x3c

I do not understand why the first page of the buffer is determined to be a bad page? Do I need to perform any initialization to the buffer pages after allocation and prior to the application calling mmap?


Any comments or advice would be appreciated.

thanks