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

Mathieu Desnoyers
2006-09-22T14:08:43Z

* 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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