meaning of vmalloc shortcut comment in fault.c

meaning of vmalloc shortcut comment in fault.c

[Bjorn Wesen]

Can someone elaborate on why it's bad to refer to tsk directly below (this
is a 2.4.5 change in x86) and why it's needed on x86 and not other archs..

What should I do for an arch that does not have a "cr3" machine register
to check with ?

/BW

vmalloc_fault:
        {
                /*
                 * Synchronize this task's top level page-table
                 * with the 'reference' page table.
                 *
                 * Do _not_ use "tsk" here. We might be inside
                 * an interrupt in the middle of a task switch..
                 */
                int offset = __pgd_offset(address);
                pgd_t *pgd, *pgd_k;
                pmd_t *pmd, *pmd_k;
                pte_t *pte_k;

                asm("movl %%cr3,%0":"=r" (pgd));
                pgd = offset + (pgd_t *)__va(pgd);

Re: meaning of vmalloc shortcut comment in fault.c

[Brian Gerst]

Bjorn Wesen wrote:
> 
> Can someone elaborate on why it's bad to refer to tsk directly below (this
> is a 2.4.5 change in x86) and why it's needed on x86 and not other archs..
> 
> What should I do for an arch that does not have a "cr3" machine register
> to check with ?

%cr3 is the page table pointer on the x86.  Changing the page table
pointer and changing the stack pointer (from which tsk is derived) is
not done atomically during a task switch.  If an interrupt happens in
between and causes a page fault on a vmalloced area, using tsk may refer
to the wrong page tables.  %cr3 by definition always has the current
page table pointer.

--

				Brian Gerst