Re: memory offline infinite loop after soft offline

[Naoya Horiguchi]

On Fri, Oct 18, 2019 at 09:33:10AM +0200, Michal Hocko wrote:
> On Fri 18-10-19 06:32:22, Naoya Horiguchi wrote:
> > On Fri, Oct 18, 2019 at 08:06:35AM +0200, Michal Hocko wrote:
> > > On Fri 18-10-19 02:19:06, Naoya Horiguchi wrote:
> > > > On Thu, Oct 17, 2019 at 08:27:59PM +0200, Michal Hocko wrote:
> > > > > On Thu 17-10-19 14:07:13, Qian Cai wrote:
> > > > > > On Thu, 2019-10-17 at 12:01 +0200, Michal Hocko wrote:
> > > > > > > On Thu 17-10-19 09:34:10, Naoya Horiguchi wrote:
> > > > > > > > On Mon, Oct 14, 2019 at 10:39:14AM +0200, Michal Hocko wrote:
> > > > > > > 
> > > > > > > [...]
> > > > > > > > > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> > > > > > > > > index 89c19c0feadb..5fb3fee16fde 100644
> > > > > > > > > --- a/mm/page_isolation.c
> > > > > > > > > +++ b/mm/page_isolation.c
> > > > > > > > > @@ -274,7 +274,7 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn,
> > > > > > > > >  			 * simple way to verify that as VM_BUG_ON(), though.
> > > > > > > > >  			 */
> > > > > > > > >  			pfn += 1 << page_order(page);
> > > > > > > > > -		else if (skip_hwpoisoned_pages && PageHWPoison(page))
> > > > > > > > > +		else if (skip_hwpoisoned_pages && PageHWPoison(compound_head(page)))
> > > > > > > > >  			/* A HWPoisoned page cannot be also PageBuddy */
> > > > > > > > >  			pfn++;
> > > > > > > > >  		else
> > > > > > > > 
> > > > > > > > This fix looks good to me. The original code only addresses hwpoisoned 4kB-page,
> > > > > > > > we seem to have this issue since the following commit,
> > > > > > > 
> > > > > > > Thanks a lot for double checking Naoya!
> > > > > > >  
> > > > > > > >   commit b023f46813cde6e3b8a8c24f432ff9c1fd8e9a64
> > > > > > > >   Author: Wen Congyang <wency@xxxxxxxxxxxxxx>
> > > > > > > >   Date:   Tue Dec 11 16:00:45 2012 -0800
> > > > > > > >   
> > > > > > > >       memory-hotplug: skip HWPoisoned page when offlining pages
> > > > > > > > 
> > > > > > > > and extension of LTP coverage finally discovered this.
> > > > > > > 
> > > > > > > Qian, could you give the patch some testing?
> > > > > > 
> > > > > > Unfortunately, this does not solve the problem.ÂIt looks to me that in
> > > > > > soft_offline_huge_page(), set_hwpoison_free_buddy_page() will only set
> > > > > > PG_hwpoison for buddy pages, so the even the compound_head() has no PG_hwpoison
> > > > > > set.
> > > > > > 
> > > > > > 		if (PageBuddy(page_head) && page_order(page_head) >= order) {
> > > > > > 			if (!TestSetPageHWPoison(page))
> > > > > > 				hwpoisoned = true;
> > > > > 
> > > > > This is more than unexpected. How are we supposed to find out that the
> > > > > page is poisoned? Any idea Naoya?
> > > > 
> > > > # sorry for my poor review...
> > > > 
> > > > We set PG_hwpoison bit only on the head page for hugetlb, that's because
> > > > we handle multiple pages as a single one for hugetlb. So it's enough
> > > > to check isolation only on the head page.  Simply skipping pfn cursor to
> > > > the page after the hugepage should avoid the infinite loop:
> > > 
> > > But the page dump Qian provided shows that the head page doesn't have
> > > HWPoison bit either. If it had then going pfn at a time should just work
> > > because all tail pages would be skipped. Or do I miss something?
> > 
> > You're right, then I don't see how this happens.
> 
> OK, this is a bit relieving. I thought that there are legitimate cases
> when none of the hugetlb gets the HWPoison bit (e.g. when the page has 0
> reference count which is the case here). That would be utterly broken
> because we would have no way to tell the page is hwpoisoned.
> 
> Anyway, do you think the patch as I've posted makes sense regardless
> another potential problem? Or would you like to resend yours which skips
> over tail pages at once?

I think the patch does no harm but we may put this pending until we
understand the mechanism of the bug. I'll dig this more next week.

Thanks,
Naoya Horiguchi