On Tue, 22 Apr 2014 04:20:22 +0300 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote:
> On Mon, Apr 21, 2014 at 06:02:27PM -0700, Andrew Morton wrote:
> > > Could you show resulting faultin_page() from you tree? Or I can just
> > > rebase it on top of your tree once it will be published if you wish.
> >
> > It looked like this:
> >
> > ret = faultin_page(tsk, vma, start, &foll_flags,
> > nonblocking);
> > switch (ret) {
> > case 0:
> > break;
> > case -EFAULT:
> > case -ENOMEM:
> > case -EHWPOISON:
> > return i ? i : ret;
> > case -EBUSY:
> > return i;
> > case -ENOENT:
> > goto next_page;
> > default:
> > BUILD_BUG();
> > }
> >
> > is that what you tested?
>
> Yes. But I wanted to see faultin_page() itself, not caller. :)
It's a gcc thing.
static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
unsigned long address, unsigned int *flags, int *nonblocking)
{
struct mm_struct *mm = vma->vm_mm;
unsigned int fault_flags = 0;
int ret;
/* For mlock, just skip the stack guard page. */
if ((*flags & FOLL_MLOCK) && stack_guard_page(vma, address))
return -ENOENT;
if (*flags & FOLL_WRITE)
fault_flags |= FAULT_FLAG_WRITE;
if (nonblocking)
fault_flags |= FAULT_FLAG_ALLOW_RETRY;
if (*flags & FOLL_NOWAIT)
fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
ret = handle_mm_fault(mm, vma, address, fault_flags);
if (ret & VM_FAULT_ERROR) {
if (ret & VM_FAULT_OOM)
return -ENOMEM;
if (ret & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
return *flags & FOLL_HWPOISON ? -EHWPOISON : -EFAULT;
if (ret & VM_FAULT_SIGBUS)
return -EFAULT;
BUG();
}
if (tsk) {
if (ret & VM_FAULT_MAJOR)
tsk->maj_flt++;
else
tsk->min_flt++;
}
if (ret & VM_FAULT_RETRY) {
if (nonblocking)
*nonblocking = 0;
return -EBUSY;
}
/*
* The VM_FAULT_WRITE bit tells us that do_wp_page has broken COW when
* necessary, even if maybe_mkwrite decided not to set pte_write. We
* can thus safely do subsequent page lookups as if they were reads.
* But only do so when looping for pte_write is futile: in some cases
* userspace may also be wanting to write to the gotten user page,
* which a read fault here might prevent (a readonly page might get
* reCOWed by userspace write).
*/
if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
*flags &= ~FOLL_WRITE;
return 0;
}
> > I suspect what happened is that your gcc worked out that faultin_page()
> > cannot return anything other than one of those six values and so the
> > compiler elided the BUILD_BUG() code. But my gcc-4.4.4 isn't that smart.
>
> Could be. I use gcc-4.8.2.
>
> > No matter, let's just leave it as a BUG().
>
> I'm okay with that.
>
> I just want to make sure that we don't step on a real bug here. I'll
> recheck everything once your updated tree will be in linux-next.
OK, I'll get onto that tomorrow.
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