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

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Sat, Apr 18, 2015 at 06:12:56PM -0400, Linus Torvalds wrote:
> On Sat, Apr 18, 2015 at 5:59 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> > On Sat, Apr 18, 2015 at 5:56 PM, Kirill A. Shutemov
> > <kirill@shutemov.name> wrote:
> >>
> >> 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.
> >
> > Ahh, yes, that does look like a more likely culprit.
> 
> That said, I do think we should likely also do that
> 
>         WARN_ON_ONCE(PageHuge(page));
> 
> in __put_compound_page() rather than just silently saying "no refcount
> changes for this magical case that shouldn't even happen".  If it
> shouldn't happen, then we should warn about it, not try to ":handle"
> some case that shouldn't happen and shouldn't matter.

__put_compound_page() can be called for PageHuge, so I don't think that adding
WARN_ON_ONCE(PageHuge) is good (, which makes every hugetlb user see the warning
once in every boot.)

What I thought when I suggested this code was that __page_cache_release() seems
not to be intended for hugetlb, but I'm not sure.
__put_compound_page() does work without this !PageHuge check which is only for
potential change in __put_compound_page().
So if everyone thinks that __put_compound_page() is stable and will never change
in the future, this !PageHuge check is totally unnecessary.

> Let's not play games in this area. This code has been stable for many
> years, why are we suddenly doing random things here? There's something
> to be said for "if it ain't broke..", and there's *definitely* a lot
> to be said for "let's not complicate this even more".

OK, so could you please try simply reverting 822fc61367f0 ?

Thanks,
Naoya Horiguchi

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