Page fault in kernel code

[cl@linux.com (Christoph Lameter)]

On Thu, 11 Sep 2014, Leon Romanovsky wrote:

> > Linux kernel memory is not page-able, but memory allocated through vmalloc
> > can still cause page fault. How device drivers using vmalloc handle this?
> Pages allocated via vmalloc call won't generate page-faults.

Kernel faults are used on some platforms in a very limited way but not
for swapping in or "paging in from disk". Have a look at
linux/arch/x86/mm/fault.c:


      /*
         * We fault-in kernel-space virtual memory on-demand. The
         * 'reference' page table is init_mm.pgd.
         *
         * NOTE! We MUST NOT take any locks for this case. We may
         * be in an interrupt or a critical region, and should
         * only copy the information from the master page table,
         * nothing more.
         *
         * This verifies that the fault happens in kernel space
         * (error_code & 4) == 0, and that the fault was not a
         * protection error (error_code & 9) == 0.
         */
        if (unlikely(fault_in_kernel_space(address))) {
                if (!(error_code & (PF_RSVD | PF_USER | PF_PROT))) {
                        if (vmalloc_fault(address) >= 0)
                                return;

                        if (kmemcheck_fault(regs, address, error_code))
                                return;
                }

                /* Can handle a stale RO->RW TLB: */
                if (spurious_fault(error_code, address))
                        return;

                /* kprobes don't want to hook the spurious faults: */
                if (kprobes_fault(regs))
                        return;
                /*
                 * Don't take the mm semaphore here. If we fixup a prefetch
                 * fault we could otherwise deadlock:
                 */
                bad_area_nosemaphore(regs, error_code, address);

                return;
        }