Dear Maintainer,
Actually i met a VM_BUG_ON_PAGE issue in centos7.4 some days ago.When the
issue first happen,
i just can know that it happen in free_huge_page() when doing soft offline
huge page.
But because page->mapping is set to null,i can not get any further
information how the issue happen.

So i modified the code as the patch show,and apply the new code to our
produce line and wait some time,
then the issue come again.And this time i can know the whole file path
which trigger the issue by using
crash tool to get the inode、dentry and so on,that help me to find a way to
reproduce the issue quite easily
and finally found the root cause and solve it.

I think if keep the page->mapping,we can even do the rmap to check more
detail info too by using the crash tool to analyse the coredump.
So i think preservning the page state would more or less help to debug.
But if it is not so meaningful,just let it go. ^_^

Thank you for your time.

Best Regards

On Tue, Nov 13, 2018 at 9:04 PM Michal Hocko <mhocko@kernel.org> wrote:

> On Tue 13-11-18 20:38:16, Yongkai Wu wrote:
> > It is better to keep page mapping info when free_huge_page() hit the
> > VM_BUG_ON_PAGE,
> > so we can get more infomation from the coredump for further analysis.
>
> The patch seems to be whitespace damaged. Put that aside, have you
> actually seen a case where preservning the page state would help to nail
> down any bug.
>
> I am not objecting to the patch, it actually makes some sense to me, I
> am just curious about a background motivation.
>
> > Signed-off-by: Yongkai Wu <nic_w@163.com>
> > ---
> >  mm/hugetlb.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index c007fb5..ba693bb 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1248,10 +1248,11 @@ void free_huge_page(struct page *page)
> >   (struct hugepage_subpool *)page_private(page);
> >   bool restore_reserve;
> >
> > +        VM_BUG_ON_PAGE(page_count(page), page);
> > +        VM_BUG_ON_PAGE(page_mapcount(page), page);
> > +
> >   set_page_private(page, 0);
> >   page->mapping = NULL;
> > - VM_BUG_ON_PAGE(page_count(page), page);
> > - VM_BUG_ON_PAGE(page_mapcount(page), page);
> >   restore_reserve = PagePrivate(page);
> >   ClearPagePrivate(page);
> >
> > --
> > 1.8.3.1
>
> --
> Michal Hocko
> SUSE Labs
>