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 Sat, Apr 18, 2015 at 05:27:49PM -0400, Linus Torvalds wrote:
> On Sat, Apr 18, 2015 at 4:56 PM, Borislav Petkov <bp@alien8.de> wrote:
> >
> > so I'm running some intermediate state of linus/master + tip/master from
> > Thursday and probably I shouldn't be even taking such splat seriously
> > and wait until 4.1-rc1 has been done but let me report it just in case
> > so that it is out there, in case someone else sees it too.
> >
> > I don't have a reproducer yet except the fact that it happened twice
> > already, the second time while watching the new Star Wars teaser on
> > youtube (current->comm is "AudioThread" probably from chrome, as shown
> > in the splat below).

I would guess it's related to sound: the most common source of PTE-mapeed
compund pages into userspace.

> Hmm. The only recent commit in this area seems to be 822fc61367f0
> ("mm: don't call __page_cache_release for hugetlb") although I don't
> see why it would cause anything like that. But it changes code that
> has been stable for many years, which makes me wonder how valid it is
> (__put_compound_page() has been unchanged since 2011, and now suddenly
> it grew that "!PageHuge()" test).
> 
> So quite frankly, I'd almost suggest changing that
> 
>         if (!PageHuge(page))
>                 __page_cache_release(page);
> 
> back to the old unconditional __page_cache_release(page), and maybe add a single
> 
>         WARN_ON_ONCE(PageHuge(page));
> 
> just to see if that condition actually happens. The new comment says
> it shouldn't happen and that the change shouldn't matter, but...
> 
> Of course, your recent BUG_ON may well be entirely unrelated to this
> change in mm/swap.c, but it *is* in kind of the same area, and the
> timing would match too...

Andrea has already seen the bug and pointed to 8d63d99a5dfb as possible
cause. I don't see why the commit could broke anything, but it worth
trying to revert and test.

Borislav, could you try?


> ---
> [115258.861335] page:ffffea0010a15040 count:0 mapcount:1 mapping:
>     (null) index:0x0
> [115258.869511] flags: 0x8000000000008014(referenced|dirty|tail)
> [115258.874159] page dumped because: VM_BUG_ON_PAGE(page_mapcount(page) != 0)
> [115258.874179] kernel BUG at mm/swap.c:134!
> [115258.874262] RIP: put_compound_page+0x3b9/0x480
> 
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-- 
 Kirill A. Shutemov

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