Re: kernel BUG at mm/swap.c:134! - page dumped because: VM_BUG_ON_PAGE(page_mapcount(page) != 0)

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Apr 23, 2015 at 06:23:11PM +0200, Andrea Arcangeli wrote:
> On Wed, Apr 22, 2015 at 12:26:55PM -0700, Linus Torvalds wrote:
> > On Wed, Apr 22, 2015 at 11:33 AM, Kirill A. Shutemov
> > <kirill@shutemov.name> wrote:
> > >
> > > Could you try patch below instead? This can give a clue what's going on.
> > 
> > Just FYI, I've done the revert in my tree.
> > 
> > Trying to figure out what is going on despite that is obviously a good
> > idea, but I'm hoping that my merge window is winding down, so I am
> > trying to make sure it's all "good to go"..
> 
> Sounds safer to defer it, agreed.
> 
> Unfortunately I also can only reproduce it only on a workstation where
> it wasn't very handy to debug it as it'd disrupt my workflow and it
> isn't equipped with reliable logging either (and the KMS mode didn't
> switch to console to show me the oops either). It just got it logged
> once in syslog before freezing.
> 
> The problem has to be that there's some get_page/put_page activity
> before and after a PageAnon transition and it looks like a tail page
> got mapped by hand in userland by some driver using 4k ptes which
> isn't normal

Compound pages mapped with PTEs predates THP. See f3d48f0373c1.

> but apparently safe before the patch was applied. Before
> the patch, the tail page accounting would be symmetric regardless of
> the PageAnon transition.
> 
> page:ffffea0010226040 count:0 mapcount:1 mapping:          (null) index:0x0
> flags: 0x8000000000008010(dirty|tail)
> page dumped because: VM_BUG_ON_PAGE(page_mapcount(page) != 0)
> ------------[ cut here ]------------
> kernel BUG at mm/swap.c:134!

I looked into code a bit more. And the VM_BUG_ON_PAGE() is bogus. See
explanation in commit message below.

Tail page refcounting is mess. Please consider reviewing my patchset which
drops it [1]. ;)

Linus, how should we proceed with reverted patch? Should I re-submit it to
Andrew? Or you'll re-revert it?

[1] lkml.kernel.org/g/1429823043-157133-1-git-send-email-kirill.shutemov@linux.intel.com