Re: Linux 4.18-rc7

[Kirill A. Shutemov]

On Mon, Jul 30, 2018 at 06:01:26PM -0700, Linus Torvalds wrote:
> On Mon, Jul 30, 2018 at 2:53 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> >
> > I have no problem with reverting -rc7's vma_is_anonymous() series.
> 
> I don't think we need to revert the whole series: I think the rest are
> all fairly obvious cleanups, and shouldn't really have any semantic
> changes.
> 
> It's literally only that last patch in the series that then changes
> that meaning of "vm_ops". And I don't really _mind_ that last step
> either, but since we don't know exactly what it was that it broke, and
> we're past rc7, I don't think we really have any option but the revert
> it.
> 
> And if we revert it, I think we need to just remove the
> VM_BUG_ON_VMA() that it was supposed to fix. Because I do think that
> it is quite likely that the real bug is that overzealous BUG_ON(),
> since I can't see any reason why anonymous mappings should be special
> there.
> 
> But I'm certainly also ok with re-visiting that commit later.  I just
> think that right _now_ the above is my preferred plan.
> 
> Kirill?

Considering the timing, I'm okay with reverting the last patch with
dropping the VM_BUG_ON_VMA().

But in the end I would like to see strong vma_is_anonymous().

The VM_BUG_ON_VMA() is only triggerable by the test case because
vma_is_anonymous() false-positive in fault path and we get anon-THP
allocated in file-private mapping.

I don't see immediately how this may trigger other crashes.
But it definitely looks wrong.

> > I'm all for deleting that VM_BUG_ON_VMA() in zap_pmd_range(), it was
> > just a compromise with those who wanted to keep something there;
> > I don't think we even need a WARN_ON_ONCE() now.
> 
> So to me it looks like a historical check that simply doesn't
> "normally" trigger, but there's no reason I can see why we should care
> about the case it tests against.

I'll think more on what could go wrong with __split_huge_pmd() called on
anon-THP page without mmap_sem(). It's not yet clear cut to me.


-- 
 Kirill A. Shutemov