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 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.

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".

             Linus

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