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

[Linus Torvalds <torvalds@linux-foundation.org>]

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).

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...

             Linus

---
[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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