Re: Linux 4.18-rc7

[Kirill A. Shutemov]

On Tue, Jul 31, 2018 at 09:29:27AM +0300, Kirill A. Shutemov wrote:
> 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.

I think not having mmap_sem taken at least on read when we call
__split_huge_pmd() opens possiblity of race with khugepaged:
khugepaged can collapse the page back to THP as soon as we drop ptl.
As result pmd_none_or_trans_huge_or_clear_bad() would return true and we
basically leave the THP behind, not zapped.

-- 
 Kirill A. Shutemov