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Date:   Tue, 31 Jul 2018 17:57:19 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Hugh Dickins <hughd@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Amit Pundir <amit.pundir@linaro.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        John Stultz <john.stultz@linaro.org>,
        linux-mm <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        youling257@gmail.com
Subject: Re: Linux 4.18-rc7
Message-ID: <20180731145718.pbyy3qkp2a2yvucs@kshutemo-mobl1>
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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@google.com> 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