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