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
