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
