Re: [PATCH] mm: teach dump_page() to correctly output poisoned struct pages

[Pavel Tatashin <pasha.tatashin@oracle.com>]

On Mon, Jul 2, 2018 at 11:59 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 02-07-18 11:27:45, Pavel Tatashin wrote:
> > If struct page is poisoned, and uninitialized access is detected via
> > PF_POISONED_CHECK(page) dump_page() is called to output the page. But,
> > the dump_page() itself accesses struct page to determine how to print
> > it, and therefore gets into a recursive loop.
> >
> > For example:
> > dump_page()
> >  __dump_page()
> >   PageSlab(page)
> >    PF_POISONED_CHECK(page)
> >     VM_BUG_ON_PGFLAGS(PagePoisoned(page), page)
> >      dump_page() recursion loop.
>
> This deserves a big fat comment in __dump_page. Basically no Page$FOO
> can be used on an HWPoison page.
>
> > Fixes: f165b378bbdf ("mm: uninitialized struct page poisoning sanity checking")
> > Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
>
> Acked-by: Michal Hocko <mhocko@suse.com>

Thank you, I will send out an updated version with a comment.

Pavel

>
> > ---
> >  mm/debug.c | 13 +++++++++++--
> >  1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/debug.c b/mm/debug.c
> > index 56e2d9125ea5..469b526e6abc 100644
> > --- a/mm/debug.c
> > +++ b/mm/debug.c
> > @@ -43,12 +43,20 @@ const struct trace_print_flags vmaflag_names[] = {
> >
> >  void __dump_page(struct page *page, const char *reason)
> >  {
> > +     bool page_poisoned = PagePoisoned(page);
> > +     int mapcount;
> > +
> > +     if (page_poisoned) {
> > +             pr_emerg("page:%px is uninitialized and poisoned", page);
> > +             goto hex_only;
> > +     }
> > +
> >       /*
> >        * Avoid VM_BUG_ON() in page_mapcount().
> >        * page->_mapcount space in struct page is used by sl[aou]b pages to
> >        * encode own info.
> >        */
> > -     int mapcount = PageSlab(page) ? 0 : page_mapcount(page);
> > +     mapcount = PageSlab(page) ? 0 : page_mapcount(page);
> >
> >       pr_emerg("page:%px count:%d mapcount:%d mapping:%px index:%#lx",
> >                 page, page_ref_count(page), mapcount,
> > @@ -60,6 +68,7 @@ void __dump_page(struct page *page, const char *reason)
> >
> >       pr_emerg("flags: %#lx(%pGp)\n", page->flags, &page->flags);
> >
> > +hex_only:
> >       print_hex_dump(KERN_ALERT, "raw: ", DUMP_PREFIX_NONE, 32,
> >                       sizeof(unsigned long), page,
> >                       sizeof(struct page), false);
> > @@ -68,7 +77,7 @@ void __dump_page(struct page *page, const char *reason)
> >               pr_alert("page dumped because: %s\n", reason);
> >
> >  #ifdef CONFIG_MEMCG
> > -     if (page->mem_cgroup)
> > +     if (!page_poisoned && page->mem_cgroup)
> >               pr_alert("page->mem_cgroup:%px\n", page->mem_cgroup);
> >  #endif
> >  }
> > --
> > 2.18.0
> >
>
> --
> Michal Hocko
> SUSE Labs
>