Re: [PATCH] Linux Kernel Markers 0.5 for Linux 2.6.17 (with probe management)

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Ingo Molnar (mingo@elte.hu) wrote:
> 
> * Mathieu Desnoyers <compudj@krystal.dyndns.org> wrote:
> 
> > >   "As an example, LTTng traces the page fault handler, when kprobes 
> > >   just can't instrument it."
> > > 
> > > but tracing a raw pagefault at the arch level is a bad idea anyway, 
> > > we want to trace __handle_mm_fault(). That way you can avoid having 
> > > to modify every architecture's pagefault handler ...
> > 
> > Then you lose the ability to trace in-kernel minor page faults.
> 
> that's wrong, minor pagefaults go through __handle_mm_fault() just as 
> much.
> 

Hi Ingo,

On a 2.6.17 kernel tree :

In do_page_fault()
  
        if (unlikely(address >= TASK_SIZE)) {
                if (!(error_code & 0x0000000d) && vmalloc_fault(address) >= 0)
                        return;

In vmalloc_fault()

static inline int vmalloc_fault(unsigned long address)
{
        unsigned long pgd_paddr;
        pmd_t *pmd_k;
        pte_t *pte_k;
        /*
         * Synchronize this task's top level page-table
         * with the 'reference' page table.
         *
         * Do _not_ use "current" here. We might be inside
         * an interrupt in the middle of a task switch..
         */
        pgd_paddr = read_cr3();
        pmd_k = vmalloc_sync_one(__va(pgd_paddr), address);
        if (!pmd_k)
                return -1;
        pte_k = pte_offset_kernel(pmd_k, address);
        if (!pte_present(*pte_k))
                return -1;
        return 0;
}


It seems like a shortcut path that will never call __handle_mm_fault. This path
is precisely used to handle vmalloc faults.

Mathieu

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