Re: [PATCH v3 1/2] mm,hwpoison: fix race with compound page allocation

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

On Wed, May 12, 2021 at 10:33:24AM +0200, Oscar Salvador wrote:
> On Wed, May 12, 2021 at 12:10:15AM +0900, Naoya Horiguchi wrote:
> > @@ -1095,30 +1095,43 @@ static int __get_hwpoison_page(struct page *page)
> >  {
> >  	struct page *head = compound_head(page);
> >  
> > -	if (!PageHuge(head) && PageTransHuge(head)) {
> > -		/*
> > -		 * Non anonymous thp exists only in allocation/free time. We
> > -		 * can't handle such a case correctly, so let's give it up.
> > -		 * This should be better than triggering BUG_ON when kernel
> > -		 * tries to touch the "partially handled" page.
> > -		 */
> > -		if (!PageAnon(head)) {
> > -			pr_err("Memory failure: %#lx: non anonymous thp\n",
> > -				page_to_pfn(page));
> > -			return 0;
> > +	if (PageCompound(page)) {
> > +		if (PageSlab(page)) {
> > +			return get_page_unless_zero(page);
> > +		} else if (PageHuge(head)) {
> > +			int ret = 0;
> > +
> > +			spin_lock(&hugetlb_lock);
> > +			if (!PageHuge(head))
> > +				ret = -EBUSY;
> > +			else if (HPageFreed(head) || HPageMigratable(head))
> > +				ret = get_page_unless_zero(head);
> > +			spin_unlock(&hugetlb_lock);
> > +			return ret;
> 
> Uhm, I am having a hard time with that -EBUSY.
> At this stage, we expect __get_hwpoison_page() to either return true or false,
> depending on whether it could grab a page's refcount or not. Returning -EBUSY
> here seems wrong (plus it is inconsistent with the comment above the function).
> It might be useful for the latter patch, I do not know as I yet have to check
> that one, but if anything, let us stay consistent here in this one.
> So, if hugetlb vanished under us, let us return "we could not grab the
> refcount". Does it make sense?

Yes, you are totally right.  I failed to properly split the patch.
-EBUSY is non-zero, so it's considererd as "successfully pinned", which is
not true.  I should've set ret to 0.

- Naoya