Re: [PATCH v2 4/9] mm, hwpoison, hugetlb: support saving mechanism of raw error pages

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Tue, Jun 28, 2022 at 02:26:47PM +0800, Muchun Song wrote:
> On Tue, Jun 28, 2022 at 02:41:22AM +0000, HORIGUCHI NAOYA(堀口 直也) wrote:
> > On Mon, Jun 27, 2022 at 05:26:01PM +0800, Muchun Song wrote:
> > > On Fri, Jun 24, 2022 at 08:51:48AM +0900, Naoya Horiguchi wrote:
> > > > From: Naoya Horiguchi <naoya.horiguchi@nec.com>
...
> > > > +	} else {
> > > > +		/*
> > > > +		 * Failed to save raw error info.  We no longer trace all
> > > > +		 * hwpoisoned subpages, and we need refuse to free/dissolve
> > > > +		 * this hwpoisoned hugepage.
> > > > +		 */
> > > > +		set_raw_hwp_unreliable(hpage);
> > > > +		return ret;
> > > > +	}
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +inline int hugetlb_clear_page_hwpoison(struct page *hpage)
> > > > +{
> > > > +	struct llist_head *head;
> > > > +	struct llist_node *t, *tnode;
> > > > +
> > > > +	if (raw_hwp_unreliable(hpage))
> > > > +		return -EBUSY;
> > > 
> > > IIUC, we use head page's PageHWPoison to synchronize hugetlb_clear_page_hwpoison()
> > > and hugetlb_set_page_hwpoison(), right? If so, who can set hwp_unreliable here?
> > 
> > Sorry if I might miss your point, but raw_hwp_unreliable is set when
> > allocating raw_hwp_page failed.  hugetlb_set_page_hwpoison() can be called
> 
> Sorry. I have missed this. Thanks for your clarification.
> 
> > multiple times on a hugepage and if one of the calls fails, the hwpoisoned
> > hugepage becomes unreliable.
> > 
> > BTW, as you pointed out above, if we switch to passing GFP_ATOMIC to kmalloc(),
> > the kmalloc() never fails, so we no longer have to implement this unreliable
> 
> No. kmalloc() with GFP_ATOMIC can fail unless I miss something important.

OK, I've interpretted the comment about GFP_ATOMIC wrongly.

 * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
 * watermark is applied to allow access to "atomic reserves".
 

> > flag, so things get simpler.
> > 
> > > 
> > > > +	ClearPageHWPoison(hpage);
> > > > +	head = raw_hwp_list_head(hpage);
> > > > +	llist_for_each_safe(tnode, t, head->first) {
> > > 
> > > Is it possible that a new item is added hugetlb_set_page_hwpoison()  and we do not
> > > traverse it (we have cleared page's PageHWPoison)? Then we ignored a real hwpoison
> > > page, right?
> > 
> > Maybe you are mentioning the race like below. Yes, that's possible.
> >
> 
> Sorry, ignore my previous comments, I'm thinking something wrong.
> 
> >   CPU 0                            CPU 1
> > 
> >                                    free_huge_page
> >                                      lock hugetlb_lock
> >                                      ClearHPageMigratable
> 				       remove_hugetlb_page()
> 				       // the page is non-HugeTLB now

Oh, I missed that.

> >                                      unlock hugetlb_lock
> >   get_huge_page_for_hwpoison
> >     lock hugetlb_lock
> >     __get_huge_page_for_hwpoison
> 
> 	// cannot reach here since it is not a HugeTLB page now.
> 	// So this race is impossible. Then we fallback to normal
> 	// page handling. Seems there is a new issue here.
> 	//
> 	// memory_failure()
> 	//	try_memory_failure_hugetlb()
> 	//	if (hugetlb)
> 	//		goto unlock_mutex;
> 	//	if (TestSetPageHWPoison(p)) {
> 	//	// This non-HugeTLB page's vmemmap is still optimized.
> 	
> Setting COMPOUND_PAGE_DTOR after hugetlb_vmemmap_restore() might fix this
> issue, but we will encounter this race as you mentioned below.

I don't have clear ideas about this now (I don't test vmemmap-optimized case
yet), so I will think more about this case. Maybe memory_failure() need
detect it because memory_failure() heaviliy depends on the status of struct
page.

Thanks,
Naoya Horiguchi

> 
> Thanks.
> 	
> >       hugetlb_set_page_hwpoison
> >         allocate raw_hwp_page
> >         TestSetPageHWPoison
> >                                      update_and_free_page
> >                                        __update_and_free_page
> >                                          if (PageHWPoison)
> >                                            hugetlb_clear_page_hwpoison
> >                                              TestClearPageHWPoison
> >                                              // remove all list items
> >         llist_add
> >     unlock hugetlb_lock
> > 
> > 
> > The end result seems not critical (leaking raced raw_hwp_page?), but
> > we need fix.